In the US, claiming authorship isn't as protected or significant as internationally; moral rights are only protected for "works of visual art". https://en.wikipedia.org/wiki/Moral_rights

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.

This is jurisdiction specific. The US doesn't have anything like this as long as you're not claiming to own the copyright. As far as I know, this is also why in some European countries you can't simply make a work public domain, as you can't give up attribution rights. In the US, you just don't have any attribution rights (though you could demand attribution as part of a copyright license).

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