Ok...
Constantin have been a passenger in this deal since around 1995 when they, with Marvel's agreement, signed over full control to Fox, as they clearly could not make an FF film remotely worthy of a theatrical release at that time.
In 1999 it was Fox, NOT Constantin, who got an extension as Fox had been dragging their heels and the reversion deadline was looming (this is why the rights did not revert although there was 12 years between the Constantin Corman effort and Fox's Story flick).
The only thing Constantin kept was a co-producer credit and distribution rights in Germany and I think Austria. Fox financed and controlled the films in full. If Fox had let deadline run out the rights would have reverted back to Marvel and Constantin couldn't have done a damn thing about it.
So now with Marvel buying that Fox contract, the FF rights will be home. It is possible Constantin might still have a claim for an 'in association with' credit under the terms agreed with Fox & Marvel back in the 90's, and maybe limited European distribution rights as before (although the latter might have been more of a courtesy than a legal requirement). If that is the case Disney can either let them keep that token gesture, cut them a check to piss off, or if they saw fit simply wait until the deadline runs out in 2022 and be done with them for good.
Whatever happens, the deals closes, the FF are home.