This is super odd, I've never seen anything like it. If you own the game it shows itself as being MacOS compatible in your Steam library and will happily download and install itself when asked. If you go digging around it's just downloading the .exe used for the Windows version, so I'm presuming it's a bad flag in the Store's metadata or something similar.

The game runs just fine via the usual avenues of people gaming on Mac systems, but I did find it interesting that Steam seems to think it's a compatible game.