That is precisely what he did NOT make clear.
There isn’t a syllable there that even suggests that the old Mass is forbidden. All Paul did was promulgate a new Missal. All that flowery language refers only to the promulgation of the Novus Ordo.
He knew he didn’t have the authority to abrogate the old Missal, and he never did. He knew that bishops were punishing the celebration of the old Mass, but he never did more than wink at this.
Meanwhile priests that did not comply were disciplined and reprimanded.
In practice, it was forbidden.