First of all, someone who only has 120 on something and 100 on COA, can only reach Moon level, not Planet.
Second of all, the fact that Kaido could get to Planet level with S+ moves (which he could have multiple of, compared to the opponent's single Z move), that'd still give Kaido a great advantage. As the opponent could only get to DPR 16 once in the whole fight, while Kaido could pretty much spam it lol.
Furthermore, Mythical Zoans are stupid enough as is, last thing they need is sun level as well.