Earlier quoted context omitted.
I wonder why not - they've always been pretty aggressive about Mac support, as you observed. Maybe the overhead required for developing a new FPS engine (which, AFAIK, Blizzard's never done in-house before, even SC:Ghost was another developer) was too much for them to have OS X support in the initial timetable.
I remember watching a video of some SC2 panel at Blizcon from 5-ish years ago, where they were talking about how flexible the engine and editor was. They showed some third person shooter level built in the stock SC2 editor, and it looked interesting enough. I bet that Overwatch's code is closely related to their other games, rather than something from scratch.
The content of each game is so radically different that the comparison might not be appropriate.