where does it say that? A 3rd party site that reports how it works whether or not it's a bug?
https://imgur.com/a/jIOyms6
Then why does Ezreal W (before you even damage them) proc aery? You can just hit with W, and not proc the dmg (aka nothing happened at all) and you hit Aery? No damage yet you proc Aery. Coincidentally it also draws Turret Aggro.
Then, Zilean bombs, proc Aery by picking them up... and you don't even put a shield on an ally if they pick up th e bomb. Picking up the bomb as an ally you get a buff (aka Aery should go), but you don't.
Then why does Karthus ult not do anything until the spell finishes doing dmg (despite debuffs and you can get hit)
the TLDR is that you have to go to 3rd party sites as Riot's explanations aren't helpful.