I dunno. There’s no Proton on Mac, so lumping Mac and Linux together when they really just mean Proton wouldn’t make much sense. There are native Mac and Linux games on Steam, and the Steam store page has icons and system requirement tabs for all three OSes, neither of which happens with Windows-only games. “We’re also releasing Civilization VII for MacOS and Linux / SteamOS” also sounds like they’re talking about actual Linux and Mac builds. It could all be miscommunication, of course. That wouldn’t surprise me.