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.
Do you have a source for that claim? This is the first time I'm hearing of a difference in pay for FH and Universität degrees there (regarding bachelor and master, old diploma system doesn't count).
In short, bachelors and masters are indeed legally identical to the ones acquired at universities, but most do not have the right to promotion. This mostly depends on the state they're in.
And they are not allowed to call themselves "Universität" (though some have rebranded to "Hochschule").
All FHs that I know do have an option for getting a Phd through them, but they themselves can't grant them. Instead, an FH can cooperate with an Universität through a sort of loophole. The Phd students spend their time at the FH, but they have two supervisors and get their Phd from the "Universität".
But it seems like it is only a matter of time until difference, too, is no more.
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.