Which is not to say that uploading something to github is necessarily asserting ones authorship (if you read the front matter of a book, you might see a phrasing such as "[Name] asserts the moral right to be identified at the author of this work."; this is intended for jurisdictions that comply with the Berne convention more fully than the US does.) It is also not an instrument of conveyance (which needs to be signed by the owner of the rights) or the same as registering a copyright. A copyright notice is no longer required since the Berne Convention Implementation Act of 1988 (effective as of 1989), but would involve the copyright symbol and not a git upload.

And in any case, the copyright notice is still intact. They did attribute the author.

Waive. And no, it doesn't but it doesn't require attribution either. So you're falling back on copyright law which is unlikely to protect you here - there's a reason people include licenses, after all.