When I say "useless" I mean in the grand scheme of a fight against a top tier, who would undoubtedly be able to withstand the time limit on King of Hell.
If Mihawk has AdCoC, which I see no reason he shouldn't, then current Zoro is only lasting as long as King of Hell can last. In a World where equals fight for days on end, that means Zoro still has a ways to go before he is on Mihawks level.
Obviously, Zoro is not yet a top-tier but he should be able to hold his own against most if not all top-tiers at this point. You need power more than anything else to fight off top tiers. And that's one area where Zoro is definitely not lacking. He was threatening Kaido and Big Mom even before CoC. He wasn't all that different from Luffy during Rooftop, and Luffy went on to match Kaido blow for blow even in the base as soon as he learned to coat his attacks in CoC.
Since CoC is directly proportional to one's strength, I don't see why Zoro'd be that much different from post-CoC Luffy when they were relative before CoC. Obviously, Zoro's not quite as strong as Luffy but he's not going to be still close to post-Udon Luffy when Luffy went on to become a full-fledged top-tier.
Luffy vs Kaido likely lasted less than 1.5 hours. Big Mom vs the duo was probably less than 30 minutes. The fights don't always last days. It's a matter of how intense the fight is. Zoro wouldn't need KoH just to fight a top-tier. Most top-tiers don't require advanced Haki to be hurt. So Zoro could choose not to use excessive amounts of Haki to fight them, which means he could last way longer in a fight. There is no known limit on Zoro's Haki when he's not using excessive amounts of it.
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"Kinnemon had better tools"
"Zoro did better "
Forgetting Zoro has enma which counters kaido hard
Even then zoro didn't do better
That's why he never was hyp3r by kairo
Kaido started seeing 5 top pirates when he got hurt by ryuo Luffy while he gave Zoro a kid approval "enough now, go sleep"
It's luffy who had Kaido out cold with ryuo g4 while ashura zoro couldn't do that ...he'll even Killer had kaido out cold.
Enma works with the same fundamental principle as G4 i.e., it uses excessive amounts of Haki. The Kuri fight already proved no amount of excessive Haki would help in hurting Kaido if your Haki isn't advanced enough. Once again, Luffy has level 3 penetration and he's also using excessive amounts of Haki when he was using G4 but Zoro was still keeping up or doing even better than Luffy.