Well, after all Kidd disnt have great haki feats. He indeed is strong. Makes ne think he wasnt Admiral tier at the end. Law seemed superior yet Kidd should be Yc1+ still, Kaido also 1 shotted Luffy when he was Yc1+.
Also, it might have been a critical hit, i really doubt Kidd can be 1 shotted if he used haki and guard up.