Most of that is holdover mechanics from her original design, where she healed on all her Autos (including Qs) by a flat amount.
The W is unstoppable (EVEN KNOCKUPS CANNOT INTERRUPT HER, AND you can release it mid air for free damage.) in exchange for the loss of 30% tenacity she used to have (and I would do anything to get back)
Likewise Aatrox's physical spell vamp carried over from his original Blood Pact/Blood Price toggle, although Riot removed Blood Price despite it being arguably the key of Aatrox's design.
Ironically, they were trying to REMOVE both of these champs from being drain tanks, and gated Vladimir's Q sustain for the same reason - No longer having access to % heal amplification in his own kit, Q does not heal him twice anymore, and overall healing extremely lowered... Unless he's in his brief passive window but the cast range is so short and movespeed steroid is not sufficient to avoid him being kited out and losing the 29% Missing health + 400 HP heal he gets ultra-late game.
I wonder if Ghostcrawler moving departments influenced this reversal of design philosophy, or if Riot just failed to solve the problems the reworks sought to fix. I will say, I adore both old and new Irelia's gameplay and visuals, but all of these champions became WORSE balance problems than before. Hardly an improvement.