All Oda said was Kaido won't be defeated because of his strong punch....like how Luffy defeated Doflamingo with KKG.....it could as well be like Luffy vs Lucci where Luffy outlasted and defeated with g2 which is not his "strongest" punch....seriously, you people misinterpret or take things out of context to fit your narrative...
besides, Luffy punch didn't become strong in a literal meaning...it is just bypassing defence via adCoC haki....his punch power didn't increase but his lethality increased with adCoC....