I believe you're correct, because they know the exact hardware they don't need to be compiled for each device unlike how PCs come on every version imaginable.

You can disable that feature, and some people do because they get tired of the constant downloads and shaders taking up space for games they haven't even played yet.

I have definitely seen my deck compile shaders, however I assume it downloads them if they are available. If you launch a game that uses proton for the first time while offline, you should see it compile.