They are the latter. Welcome to the San Francisco bubble, where you think everyone uses iPhones and Macs, because in your world everyone does. It's the same reason Twitter has a Mac app but not a Windows app. They build these things for themselves (and arguably they should).
It's not outlandish or absurd to design something you might want, but it is a bit silly to ignore a larger market with the same underlying need.
> 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.