Tanks can go into content with DPS gear and clear it with the bar set that high from the start. You'll probably have an easier time clearing, since healers can tune their GCDs with tank HP X in mind.
But the tank will have say, i210 leftside avg going into content dropping i240 leftside, their HP would be X-3k maybe, so not enough initially. Like for example, going into 3.2, supposing people go outright with their i210 avg gear, without getting crafted or tome stuff first, their HP would be much lower than it'll eventually be with better weapon/body/legs and other leftside parts, even if they never get VIT on accessories