3 of those are juggernauts, not tanks. And Ornn has been buffed out the ass repeatedly to the point where he fulfills the role of both.
Tank metas only exist when tanks are allowed to do relevant damage in addition to being nigh-unkillable (see: old Sejuanni, Zac, Nautilus top, current Ornn, Kench; also note that, with the exception of Nautilus, all of these tanks have % max HP damage in their kit somewhere). If there IS a tank meta upon us, it will come about because of the Sunfire buffs. Tanks can't be allowed to have the best of both worlds, otherwise they start crowding out bruisers and juggernauts, and the line between classes becomes undefinable.