Hey everyone, just a quick little fix I made today...
Basically, for STS on macOS (I prefer to call it OSX but anyways...), I accidentally had the game listed as 32-bit only, even though it's a 64-bit application. Later versions of macOS dropped support for 32-bit applications, so I think Steam was telling Mac users that the game wasn't compatible for their systems, even though it was.
So basically, now people who're updated to the latest version of macOS can play STS. Sorry about that!