Its made clear by both Kid and Luffy that the super nova were hurting them through out the fight. They were just such monsters that it took a long time to show. Also, no one in the OP verse has ever seen two Yonkos in combat together. For 5 rookies to fight them and get the upper hand (separating them) should be a big deal.
We know that roofpiece was designed later . Oda had no intention about it firstly. And after roof , Kaido did not show any hurting indication even his scar was shown minor effect by Oda.
Secondly , this statement would be believable if kaido did not fight 40 more chaps after roof . That means Kaido did showed lots of feats rest of fight. (drunk fighting, future sigh, haki blusting, magma dragon mode etc etc.) Based on manga , Looks like kaido was not affected by roofpiece part.