The existence of a tag does not automatically make something on-topic.
Be that as it may, yes, Github is, among other things, a Web application. The trouble is is that it's a tool used almost exclusively by developers. I don't think that it's necessarily off-topic here, but you, and future readers of your question, are far more likely to get a useful answer from the experts at Stack Overflow. That's the primary motivation in migrating to Stack Overflow. We've had a number of questions on Github here recently, and by and large they are going unanswered, because we simply don't have the Github experts here who can answer them.
Since the question's migration has been rejected from Stack Overflow, I've voted to re-open the question. Perhaps with some more questions coming in we'll be able to attract some experts on Github who will answer the questions here and make this a viable resource for such questions.
As for the larger conversation about whether Github (or the web app of any code repository) belong here at all, I'm kind of on the fence. (We've mostly decided that Google Analytics questions almost always belong at Webmasters, but I don't think this is as clear.)