Earlier quoted context omitted.
It is rather disappointing that github heads down the Mac OS path while they base their company around a concept which is rather strange to most things Apple - openness and sharing. Writing for Mac OS first also makes it harder to port to other platforms as it is a rather idiosyncratic platform. If they insist on pushing Mac OS AND want to be seen as a good free software citizen they might want to consider putting so…
I'm making an assumption that the majority (if not entirety) of GitHub employees use a Mac, so why wouldn't they make an editor for themselves on a platform that they use? I doubt they're doing it to bolster the OSX ecosystem as much as meet their own needs.
Remember what happened when Linus decided he wanted use BitKeeper to maintain Linux? Also remember how that worked out in the end? Free and/or open source software and closed, proprietary companies often make odd bedfellows.