Same story, just change contribute by distribute. As long as X patent holder doesn't distribute the software, GPL does just nothing about the patent.
Yes but the discussion was whether one distributor could hypothetically get an advantage over the others to become the sole distributor. With Mir, Canonical can relicense, and then only they can sell a closed source version (but everyone else can still make open source versions). Canonical can not assert a patent to prevent other people distributing Mir. With Wayland, any distributor can assert a patent and then they become the sole distributor.
External companies who have nothing to do with the project attacking the project with patents is a different issue to that of a distributor trying to gain advantage over others in order to become the sole distributor (the accusation levelled at Canonical).