I suspect Apple's intent is to leverage the massive interest in game development on their other platform, iOS, to spur game development on Mac. This feels fairly evident with the Apple Arcade push and Catalyst allowing for relatively easy porting from iOS.

I doubt that Epic would have bothered targeting Metal in UE4 if it were macOS only. With it, building games for Macintosh for a great deal of developers is fairly trivial. Unity also targets Metal, as far as I am aware.