Does it actually work though? Ironically my experience is that windows api + proton are a more stable target than anything linux native. Even valve doesn't get it always right when shipping linux versions of their own games. See https://steamcommunity.com/sharedfiles/filedetails/?id=30358... for example.

You mean the Steam Linux Runtime?

From a quick search this is the best description of it I found: https://github.com/ValveSoftware/steam-runtime

Valve funds SDL development, I think