Tbh neither of them should have had this happen, it was a common strategy for illaoi to put tentacles in bushes so when i'd get ganked i'd run back to a bush because i know there is a tentacle in there that they can't get to and that i can use to my advantage. Gangplank does the same thing it's just that he actually needs to be able to do that, he's extremely squishy and his laning phase is bad so being able to place things in a bush give him an extra edge.
If anything you should learn tendencies of certain champions and understand what they're doing. Illaoi placing tentacles in a bush is very common, same with gangplank and his barrels.
Multi-Hits or not it's a valid strategy that the opponent should learn to play around.
PS: Chances are if you tried to fight an illaoi and she started running, she was going to kill you anyway, she just chose to run back to part of the lane that she had control over. That's literally how she's supposed to be played, i do it constantly and i don't really consider it cheese it's their fault for falling for my bait.