And that's a very good reason to rule in certain way. But I was trying to outline what the rules actually say. And rules do not use these terms in plain English sense. Plain English meaning and D&D rules definition of 'humanoid' don't align either.
The whole point of 5E was to use plain English and move away from that kind of thing though
Also, if you think that a slain zombie becomes 'a humanoid corpse' what happens if one casts revivify or raise dead on it?
It comes back to life as a Human (presuming the other preconditions for the spell are met).
If you cast Raise Dead on Timmy (a formerly Human) but now undead Zombie, nothing happens. If you cast it on Timmys corpse (a former Undead) it works just fine, because that Corpse is not undead. It's just... dead.