The MIT/BSD licenses are explicitly compatible with re-licensing derived works in this way. The GPL is not.

They may be compatible but you cannot remove someone else's copyright and license from their source code. They specifically say you must include the license.

Wix is still in the wrong though. Saying your vendor is in violation of their license doesn't give you any right to violate their license.

I anal but I don't think this nullifies WordPress' GPL.

Wix may still be in the wrong, but WordPress is definitely in the wrong for removing copyright and license. IANAL but both of these companies would look awful funny in court because disregard for other people's licenses doesn't help your case when someone disregards your license.

As far as I can see the files that were originally under MIT still have their original copyright statements intact, as comments above illustrate. Which files are you alleging have had their credits removed from?