Yes, they have an advantage but that's not anticompetitive. That's just the reality of the world.
> It's just practically difficult by the nature of the problem.
What I said here is why. It's not easy to allow external parties to "just download the entirety of Github." It's not unreasonable to rate limit your infrastructure, especially if the person using it isn't paying you money.
The fact MS can train on the code more easily is irrelevant here. It's possible for a third party to download the code, it'll just take longer.