Zoro choosing to block the attack on his own to help the rest get away in the last second doesn't imply that his defensive prowess is above the other 4 Supernovas combined, or that his defensive prowess is clearly by leaps and bounds above his Captain for example.
Kid, Killer and Law probably get insta wiped out on their own, but I think Luffy would be able to hold the attack for at least as much as Zoro if he used Tankman + Advanced CoA, and were to put everything he has into defending.
Big Mom and Kaido being surprised by their attack being momentarily stopped by Zoro, if anything, implies that they underestimated the Worst Generation (and the power of plot too).
Zoro thought that the five of them (including himself) wouldn't be capable of stopping Hakai even if they worked together, so he decided to work as bait to gain time for Law to do his thing and don't put the rest into danger.