Once the rights have been granted to copy, distribute and modify the program any attempt to revoke those rights is imposing further restrictions, which as the quoted section says is forbidden.
Except that the licence is a contract from the copyright holder to the licencee. The copyright holder has the right to change the terms of the licence, or add new licences at any time. It may be possible for Oracle, as the new owner of the MySQL copyright, to revoke the GPL licence.
And who's going to stop them?
Since the FSF doesn't own the copyright, it might not be granted the right to sue Oracle for a breach of the licence. Besides who'd want to take a behemoth like Oracle on in court anyway. It would be like Lionel Hutz standing up against Burn's cadre of high priced on-retainer attorneys. At the very least it would drag on for years while Oracle systematically destroys MySQL. Even if the FSF won, there'd be nothing left.