GitHub's TOS is subject to change so that doesn't hold water. Tomorrow they could claim in their TOS you owe them your firstborn if you upload code to GitHub but that doesn't mean that you are bound by those terms because they cross the reasonable expectation of what you are signing up for. Granting Microsoft a blanket license to use your code in any way they see fit was not a part of the deal for GitHub, and as far as I know it still isn't for code that you claim copyright on. If you release your code into the public domain or use a license that is so permissive that anybody can use it at will, even without attribution that would make it fair game.