In the Titan EX case, if he's not on add duty all he really needs to do is stand there and swap. Even if he's horrible with aggro the other tank can compensate by not attacking at all after provoke.Yeeeeeaaah, but I think that's more for fights where the tanking mechanics are simpler, if not downright stationary. I can see a good healer carrying an undergeared Tank through Titan HM for example. All the Tank has to do for that fight is stand still and hold enmity after all. It helps a ton if the Tank knows how to properly mitigate damage, but even that is optional if the healers are good enough.
Buuut, for fights where enmity has to regularly be handed off, adds need to be picked up and kited to specific areas, bombs must be destroyed and landslides must be lured, I can see no practical way to carry Tanks in those fights if they don't know the mechanics.
I mean, sure, they could be carried in the sense that they may be undergeared for the fight, but I still think they need to know the mechanics, and execute them properly.
It's true you can't carry the tank in the "he dies at start and we win" sense, but in the "he's awful and other players are massively compensating for his inadequacies" sense there are plenty of opportunities in every fight.
It's not like bad tanks disappear at high level play. You'll see plenty in Ifrit EX just like you did in Garuda and Titan.