+Benedict said in his 2007 motu that Quo Primum was still legally in force. He never said the new mass was legal; it was only implied. But by explicitly declaring QP to be in force, which disallows any alternative rite, then he has explicitly declared the new mass to be illicit.
Just because the V2 popes pass a law doesn’t mean it’s licit. Plenty of states pass anti-gun laws that are later declared unconstitutional.