Garchomp and Tyranitar themselves are in OU, but their mega forms aren't actually used very often. They can't implement policies like "You can use Garchomp in UU, but only if it's holding the mega stone, and only if you mega evolve it immediately at the first available opportunity" (after all, you could have gone mega with something else earlier in the same match!) so, without the ability to allow a particular Pokemon+item combination in a lower tier than the Pokemon alone ranks at, the term they use for that case is "OU by technicality".
