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.
Just distribute the code for local execution? Sure, it's less accessable for the target audience, but it is more transparent.
But what else is new? Most users are willing to sacrifice privacy and security for convenience. That's how we got into this whole mess.