You're assuming no game studio would test their windows executable on proton, just because they develop on Windows. If there's non-trivial market share to capture by being "Deck verified" I don't see why that would be the case. Game devs develop on Windows for PlayStation, Switch, mobile etc. At least with proton they don't even need to cross compile.
Microsoft test on and make considerations for Proton for a bunch of their releases. Sony do too. Cyberpunk has specific graphics presets, Baldurs Gate 3 has a native executable, indie games often do too.
There's outliers, it'd be fair to say EA don't give a damn. But a lot do and you can't handwave away Microsoft and Sony as small fish either.
