Well, I'm not having much luck easily finding the forum threads that I was referring to, but I can explain it simple enough. What the law actually says is,
The type of threads on guns like this M&P-22 , The Walther P22, The Sig or GSG 1911-22 , and so on , they are not threads as defined in the law as being capable of attaching any of those things, and that's why they are able to be sold in NY. Instead what you have on these guns are internal threads which hold on the barrel bushing. When the pistol is in battery, the barrel bushing (which is on the threads) is completely inside the slide. The gun wouldn't actually function properly without the barrel bushing attached to those internal threads. The threads also have a very fine thread pitch which is proprietary to only the the barrel bushing and would not support any kind of attachment.
However, all of those guns do have aftermarket barrel bushings available which can convert them to have external threads which would support an attachment. So, if you were to buy and install one of those aftermarket adapters, then you would have an 'assualt weapon'.