They're not ignoring a larger market, they're ignoring a smaller one:
> Beyond that, one-third of our traffic uses Macs. It's not a small market for us.
> Beyond that, one-third of our traffic uses Macs. It's not a small market for us.
And as others have pointed out, the market doesn't just consist of GitHub users, or even Git users. In this case, it's developers who use an RCS and need a good desktop application to go with it.
In any case the point may be moot; judging by the rest of the comment you quoted they're not ignoring other platforms at all but rather just using Mac OS as a first step, which I can understand.