Correct. People are losing the bubble on this.

All S&W M&P22 Compact barrels are threaded. All of them. The first one introduced had a "cap" threaded on over the threads that looked just like the end of a normal barrel. It is the same diameter as the rest of the barrel, albeit with a slight "bulge" to act as as sort of a bushing for slide lockup. The slide can be removed with this cap in place. In fact, if you didn't look for it, you might not notice the barrel is threaded. Here's a picture of the barrel with the cap very slightly unscrewed.



Another picture, screwed on tight:



And here it is removed.



This is pretty common, and the same way guns like the GSG 1911 .22 and P22 are done. With this model (the first one I bought), I had to unscrew the cap and purchase a suppressor adapter that threaded onto the barrel. The standard thread for a suppressor is 1/2"-28, and that is too large to fit through the hole in the slide for the barrel, not to mention the shoulder provided for the can to index against. Here's what the adapter looks like:



Here's a comparison of the factory "cap" (top) and a suppressor adapter (bottom):



Notice that the suppressor threads are exposed. To protect those, you use a 1/2"-28 thread protector. It either comes with the adapter or you buy it. That's what people are advertising when they say 1/2"x28 thread protector on Amazon, etc. It's a little ring, usually knurled, and with the adapter and thread protector in place, it looks like this:



Once that adapter gets screwed onto the barrel, there's no way it will go back through the slide. Since you have to move the slide forward over the end of the barrel in a fixed barrel gun to remove it, you see the need to remove the adapter if you want to, say, field strip the gun for cleaning.

Since I bought on of the "original" ones, I had to buy my own suppressor adapter (and wrench, but that's a whole other story), unscrew the cap, and replace it with the adapter. Some time later, S&W decided to offer their own, from-the-factory suppressor ready version (the second one I bought). It comes with the suppressor adapter in place of the "cap."

So....if you bought an "original," with the cap, you can remove the cap and replace it with a suppressor adapter, if you want. If you bought a "suppressor-ready" version, and want to get rid of the suppressor adapter (for whatever reason), you have to get a cap specifically designed for the M&P22 Compact. It has to fit the threads AND barrel profile of the Compact. Possible sources include 1) directly from S&W if they have and will sell them; 2) from a M&P 22 Compact user who replaced his with a suppressor adapter; 3) have one custom made by a gunsmith or someone handy with a lathe. Shouldn't really be too hard.