Just the absence of a license generally means the creator has all right reserved by default. You don’t need a license in every file because in much of the world copyright is given by default to the creator. A licensed file is permission to do something with that copyright material.
He had a top level license file that presumably applies to all files. He would not be the first to do that and will not be the last.
That said, if Microsoft had forked before the LICENSE was added or stated somewhere, they were reusing all-rights-reserved code, which is definitely copyright infringement. Again, I am not a lawyer.
