he didn't fall, he was thrown at falling Oars... and yes he shouldn't take damage from small fries' weak attacks IF its intended for him to Defeat the strongest character in the series right away... if Kaido is his opponent then Killer doing that to Zoro was bad, Queen doing that to him was Bad, Apoo Doing that was bad, Hawkins doing that was bad, all the people mentioned are one hit KO for Kaido and you want that Kaido to be defeated by a guy who was portrayed like this ? why not Sanji then instead of Zoro, Sanji also got broken leg from vergo, beaten by flamingo, beaten by Judge, beaten by BM and now straight up Defeating Kaido, no problem there as well then huh
Alright let's look at it this way,
Do you realise that there is no proof that Zoro can tank one of his own attacks. Like straight up if another Zoro were to throw a 3000 worlds attack on Zoro's abdomen, he will most likely get torn in half. This is something unique to Zoro among the monster trio. He's damage output vastly outmatches durability. Unless blocking is involved. There's a case for Zoro being able to block his own 3000 worlds attack if he manages to intercept with his swords and haki.
See most people don't expect Zoro to be able to tank a thunder bagua to the face or something. What's more likely is that at the very least he'll just block it with his swords or maybe even parry if he's that good. And of course the main thing has always been "Can Zoro even damage kaido?" And given the display that fuckin scabbards can damage kaido, I think it's pretty obvious Zoro can too. And remember, Zoro's damage output being vastly above his durability has always been how it's been.
So again, Zoro was tossed like a hundred meters and took damage but that's because that's what he is. He can't BLOCK A FALL. Get it? He's biggest source of defense for example in the kaido fight would be his ability to block or parry and that doesn't apply to falling does it? Like he can't block or parry the actual ground or gravity itself.
In fact we've even seen this before. Zoro vs Pica. Zoro can just cut Pica's entire mountain body and yet pica says "All I have to do is knock you off the rocks so that you fall to your death". Because of course that would damage him severely. It's a fall. Pica could've killed Zoro with any single one of his stone golem punches for the entire fight but Zoro was able to parry all of Pica''s attacks when it was Zoro vs the Golem. And yet at the very end, simply falling could've taken Zoro out because you can't block or parry a damn fall.
Understand now? You are using luffy mechanics to dictate what scenarios Zoro should or should not be able to survive. You even made a mistake since Apoo literally knocked luffy out yet you're claiming taking damage from Apoo automatic means you literally cannot compete with kaido in actual combat where all powers and techniques and environmental advantages and strategies apply