Yep, oddly enough they made the game engine themselves like they did with almost all of their other games; apparently this time they didn't make it cross-platform.
Pretty strange considering the other (arguably more complicated) engines were supported from day 1, even their unreleased FPS (Ghost?) was going to be launched like that.
If they truly cared about the Mac, they would release Overwatch for macOS. It runs just fine on modern Macs via bootcamp.