That was almost a casual attack from Mihawk. There are good chances that named attack from Zoro is already above that air slash, afterall it is true that Mihawk destroyed casually from km afar the special ice of Aokiji yet Zoro too cut an horn of the skull. From a story perspective it has also to be stronger, it wouldn't make sense to have an almost casual slash of Mihawk to be already able to injure Kaido/BM (unless if Oda somewhat wants to play Shanks and so Mihawk as much stronger than Kaido/BM but that doesn't make much sense either).
Thinking back about it the attack that Mihawk sent to WB to probe him and that was stopped by Jozu is most likely stronger than the wave splitter, at least there was some amount of effort and you wouldn't send an half assed attack to somebody you want to probe ("see the gap between me ,Mihawk, and that man, WB") and is recognized as the WSM.