Why does ekko's ult heal him?

If time travel for him is inhabiting a previous state of being, then yes... it does

It didnt reverse the damage completely but it must have helped some since he did a barrel roll after clutching his supposedly broken arm. Also why bring up the cinematic like anything he does is based on that. He doesnt turn back time at all.

for the lulz

He's not traveling back in time, he's rewinding the state of his body to a recorded point. Tower damage, enemy status, the timer on the match, none of that changes.

Think of it like the movie..."The Butterfly Effect" to some extent.

It reverses part of the damage

Also, most of the damage is because his mind REMEMBERS him getting damaged, and thus thinks he should be damaged- the mind is a powerful thing.