I don't think this is true. GitHub (the website) became very successful with virtually no feature overlap with the git CLI. I suppose you could say that GitHub pull requests and forks are a replacement for git's built-in email-based workflow, but that's not really what people complain about when they say git is hard to use (and it's not like the other DVCSes had obviously better solutions).
Lol wut. Github's success is in being a remote code repository.
So your claim is that GitHub would be equally successful if their website was just information about how to push to a git remote hosted by them?
SourceForge was pretty successful back in the day and they were pretty bland compared to GitHub.
GitHub's UX is so incredibly terrible. Pull Requests are horrible -- even mailing lists are better and that's impressive.
GitHub's UX is so bad it has indeed generated a whole industry of out-of-band review tools.