The RWTH has never been a FH, though. It‘s „Technische Hochschule“ for the same reason why the University of Stuttgart was a „Technische Hochschule“ for a long time: traditionalists insist that a university must have medicine, law and religion.
And if you don't trust Wikipedia, the RWTH refers to itself as an "Universität", see here for example: https://www.rwth-aachen.de/cms/root/Die-RWTH/Aktuell/Hochsch...
Doing so would be illegal if they weren't one, so I don't see where the claim that they're not comes from.