Here's the deal. It's very tough, though not impossible, to purchase a standalone LDC nowadays. Most retailers permanently fix them to their guns before shipping them out to their customer. How do I know? Because I have a few - Daystate Regal XL, Brocock Compatto - which are near impossible to remove unless a lot of heat is applied, potentially damaging the shroud or the included LDC.
A few years ago, I found an independent retailers in Arizona that was selling quality Weihrauch LDC's for .177 up to .25 calibers so I jumped on the deal considering that they were standalone and shipped to my house. Now the hard part was tracking down adapters for my guns, like my Air Arms S500's, Taipan Mutants, often times going on Ebay and hoping to find an overseas seller willing to sell me the adapter without some restriction in place.
During this time, Neil Clague was very active on the forums, and most people knew about him. He is capable of making some really custom LDC's for whatever gun you have. The problem was that because he built each LDC as a custom product, the wait time was long depending on how busy he was, and his prices varied based on what you wanted. Unfortunately, most customers, including myself, didn't know whether an LDC or custom shroud would be the better option. He eventually left the forums as far as I know but is still in business. The problem is that you can't just go to his website and order an LDC and have it in your hands in a few days and test it out.
So here comes Donny. He has been making some custom LDC's as well but they are a universal fit for the most part, using the 1/2x20 UNF fitting. The Fatboy LDC comes to mind. His LDC's were easy to purchase off his website, arrived to your door in a few days, and were quality products able to compete with the big boys from Weihrauch and Huggett. As his popularity increased due to his great customer service and good products, he decided to go full in on this niche market and started to come out with what you see on his website today. He's not only making standalone LDC's, but also making adapters, custom LDC's, shrouds, etc. which no one else is doing, especially here in the USA. Overseas in the UK and elsewhere where there are no "perceived murky rules on LDC's for airguns" like here, you can buy them easily in a gun store.
His LDC's are top notch. I own several, including the Tanto, Sumo, and Ronin. His fast shipping, great customer service, constant product development, and listening to customers, makes him the guy to deal with in the future.
I think the reason you see his products for sale on the forums is that people try out his LDC's, figure out that it may not be what they wanted, and ultimately go back to purchase a different version. Nothing wrong with that.