> By uploading your content to GitHub, you’ve granted them a license to use that content to “improve the Service over time”, as specified in the ToS.

That's nonsense because they could claim that for almost any reason.

E.g. assume Google put the source code of Google search in Github. Then Github copies that code and uses it in their own search, since that "improves the service". Would that be legal?

It's like selling a pen and claiming the rights to anything written with it.