His scaling with time bombs combined with the base damage is enough to deal significant damage; in lategame teamfights, it also forces seperation in enemy movements, because enemies are near the bombed target they'll be in risk of heavy damage; this can make you able to focus down individual opponents with greater ease; not to mention, his ultimate has some very nice utility aswell, and at max W and E he can constantly have a time warp buff on.
I've played Zilean ranked and curbstomped before; I once played with a hecarim; he ulted while I bombed the enemy team, and gave enough time so that the bombs hit all 5 of them; we owned that teamfight.
The problem with Zilean, though, is that he's not the best at dueling in lategame because trying to cast a double time bomb on an enemy will put you in higher risk of being exposed to enemy fire.