No, the RWTH is indeed a university. Many (old) THs were granted the status of "Universität" in the 70s and 80s, see https://de.wikipedia.org/wiki/Technische_Universit%C3%A4t

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.