The way I like to think about it is that Proton essentially provides a standard, stable API across both Windows and Linux for gaming (Win32). We typically talk about it as a translation layer, and it is, but also to some degree it’s also “here’s an implementation of Win32 for Linux.”
If game devs can, say, buy a steam deck and know their game works on it, that means it’s gonna work on other steam decks and probably most Linux machines. It’s making it easy for devs to test and develop for Linux, even if it’s not really “on Linux.” Copy the Windows files to the steam deck, run your release checklist, and you’re good to go.