It doesn't matter that it theoretically all happen in the browser. You can serve different versions to different IPs etc. Every heuristic in me would be screaming don't use that if I would have a need for such tool.
The only way an attack vector is possible here is if you think GitHub themselves would maliciously inject an altered version of the code in the repo, and even then you'd be able to see the code and network requests in your developer tools.