So this is actually a harder question to answer than you might think.
You see, in firearm barrels, there is power to spare, so projectiles fit TIGHT. This is one of the reasons why you rarely see firearms with chokes, as they don't need it. The "choke," where the bullet is squeezed down to its final size, is at the back of the barrel just ahead of the cartridge and is called the "throat" or "leade" depending on who you ask. This leade can be cut from the breech end by the reamer while it cuts the chamber, making it relatively easy to do.
In airguns though we're short on power, so we want that pellet or slug to slip easily down the length of the barrel. The result is we have comparatively loose bores, which we tighten up at the muzzle to give the projectile that final consistent vector and we call it the "choke." Making this is not easy. This is part of the reason why our guns are so picky when it comes to ammo diameter, especially when it comes to slugs which deform less readily than pellets and don't have the skirt expansion to achieve that little "fudge factor" to bore fit.
So "are STX barrels better" is a comparative question, are they better than what? Single point cut rifling also starts with just a polished tube, where a carbide tooth is dragged up and down via a CNC machine until the grooves are cut and the lands remain. This is how most of the competition winning barrels out there are made today. There are a lot of possible reasons for this though, including the fact that it makes custom twist rates, including things like gain twist rates, cheap and accessible as one-off barrels. So the best shooters, who are developing their own loads, can test a dozen or more rifling profiles, all of which are custom, and go with the one that works best. It doesn't necessarily make it innately better, but it makes it the cheapest way to accomplish their end goal. And, to be clear, these barrels aren't cheap, but cheaper than custom buttons or mandrels. So button rifling is what I believe Lothar Walther uses for their airgun barrels. With this, a tungsten carbide button, which is shaped like the rifling, is forced up a polished tube squeezing and deforming the metal thus forming the rifling profile. This is, generally, thought of as the cheapest way to produce barrels. And it probably is actually a cheaper way to manufacture barrels, particularly if you're not super picky about performance, because the machine itself is basically a hydraulic ram and a lot of the cost is in the buttons themselves. The buttons, you see, wear out. And as they wear, the barrels they produce change. So every single barrel will be a little bit different. Using a button to form a choked barrel also presents major challenges particularly in regards to consistency. And, finally, you have cold hammer forging. Here either a short, or full length, mandrel which is the opposite shape of the bore is stuck inside an over-size metal tube which is then hammered on all sides down onto the mandrel. The result is a bore formed exactly over the mandrel. The mandrels themselves are quite expensive, but the resultant barrels tend to have exceptional mechanical properties. The CHF machines are very expensive, but the barrels they produce are "hypothetically" the best, although in practice they tend to be the best on fully automatic weapons and are less common on precision rifles. In theory though, forming a choked bore via this technology would be easy, it'd just require a very expensive custom mandrel to do it. CZ typically uses CHF barrels.
So it is at this point I want to note that all these different technologies are employed because they are the cheapest way to achieve the end goal of the resultant barrel. Different factors are involved in all cases, however cost is always a factor and so it is rare to see something more expensive for expense's sake.
Now FX's barrel machines are..... well a closely guarded secret, however it isn't as simple as just squeeze a tube. They almost certainly are high precision CNC machines which are doing things a bit more complex than just giving the tube a squeeze. I'm guessing, by looking at the barrels they produce. that the actual squeezing points are not the full length of the barrel, not even close actually. They shuffle the barrel and squeeze as they go, which allows them to dynamically vary the twist rate, bore diameter, and choke. This has real potential for both rapid development of new barrel specifications and exceptional barrel performance. It also seems likely to be fairly cost effective, although probably actually is more expensive than button rifling particularly if you're willing to use your buttons fairly long and hard. It is probably more consistent than button rifling as well. Although there is no way to know for sure.
In short though, it appears the best airguns out there in benchrest conditions run either LW or FX barrels. Any one competition may suggest one or the other is better, so in general I'm inclined to say the data is inconclusive. I would add though that there are confounding factors. To my knowledge there is no LW barrel to compete with the FX .30s, if there is anyway nobody is using it, so when the wind picks up the FX guns start to dominate as their barrel manufacturing technology has allowed them to develop a lot more competition winning barrel profiles than LW/Daystate. Meanwhile Daystate is, by the accounts I've been reading online, still working to get a grip on .25. I've not seen anything about slugs from Daystates being competitive with FX. So, again, you have to frame what "best" means. For now I'm just pleased as punch with the whole FX interchangeable barrel and liner system, because it makes it easy and inexpensive to change calibers, twist rates, chokes, etc. And, as new technology rolls out, my gun won't be left behind because for a mere 100$ I can be back at the front of the pack. So in that regard, it definitely is the "cheapest" way to keep the best barrel in your gun.
Regarding Utah Airguns' comments on slugs and slug accuracy, this is a massive can of worms. The short version is I think they're trying to cover their butts, because getting a gun to run slugs well is more than just "buy this liner and a tin of slugs." So, from a retail perspective, it is incredibly important to set modest expectations for your customers otherwise they'll be very very unhappy. This is particularly the case in regards to accuracy, because shooting is a mind game and if you believe your gun is shooting poorly you'll both have confirmation bias (seeing the fliers rather than the groups) and you'll start just shooting worse.
Regarding how accurate slugs actually are out of a LW barrel compared to an FX barrel, things are still early, but I don't think LW or CZ have really started making any slug barrels to attempt to compete. So the smallbore slug competitions we saw at RMAC were just an FX showcase. There are all kinds of weird claims out there regarding slugs though, like epicyclic swerve, a phenomenon Matt Dubber claims to observe with slugs but which it is hotly debated if it is possible/exists and if so by what mechanism? You can read more about that here:
http://www.appliedballisticsllc.com/Articles/ABDOC104_EpiciclicSwerve.pdf The point regarding slugs though is simply that we're relatively early in developing this technology in airguns. So there is a lot of speculation, contradictory information, and uncertainty out there. I wouldn't read into UA's comments there too much.
Whew. What a post. I hope that answered your question and then some.
