They were referring to all the lasting damage they had been inflicting on Kaido since the beginning of the battle. They couldn't believe that even after all that, they could still not trump Kaido, but as Luffy pointed out, it was actually working, hence him trying again and again. Furthermore, in hybrid form, he has his base speed with his dragon powers, so it's much more harder to land hits on him than in dragon form(hence Luffy getting sent flying in repetitive attempts), and especially even more so with Big Mom as a partner backing him up, hence the necessity to seperate them
We know hybrid doesn't have higher durability than dragon form from the simple fact that when we actually saw it on panel, hybrid Kaido chose to dodge a regular red hawk because it would hurt according to Luffy himself, hell he is even getting injured by Law's injection shot.
Nothing suggest he has higher durability in hybrid since as stated once again the durability comes from his fruit due to dragon scales