"an error occured while launching the game: invalid platform" occurs if I try to install from a web browser sending an install signal to the steam client.
on the Steam client directly highlighting Bionic Bay brings up a client page with the install button greyed out and the phrase "available for [windows symbol]"
Proton and wine exist, it shouldn't be possible for steam to refuse to install a game, granted there are never guarantees Proton or wine will work but Steam shouldn't prevent us from trying especially since many games that won't work with proton will work with wine.