He's a melee ADC. Like Yi or Trynd. They're not assassins.
Yi and Yasou are essentially assassins that finish their QWER burst rotation with a crit to the face.
Also - it begs the question, what exactly is a melee ADC?
For all practical purposes, Yasou fills exactly the same roll as an assassin like Fizz or Zed. He doesn't siege towers like a marksman, he doesn't pew-pew things from range like a marksman, he can't pick off enemies at range like a marksman. He goes balls-deep and wombo-combo's things to death. Where Yasou crits, Fizz get's a Lich Bane proc off, Zed smacks someone with the same skill twice.
DotA also just uses "carry" to mean "scales well with items". They apply it to a solid number of fighters, assassins, and mages.
For Yi and Yasou to be different than Zed, you mostly just mean "scales well with crit" as the definition of carry - which doesn't accurately describe how those champions are actually played.