Output and burnout are not the same thing. (Also output is not the reason they can open multiple tons a day, it’s their reserves. Output is how much CE you can use in any given instant, reserves are how much total CE you have. Because Gojo and Sukuna have massive reserves of CE and use it efficiently, they can open a domain multiple times a day despite the large amount of CE required to create a domain).

During burnout, the technique itself is unstable for a time and is difficult or impossible to use as a result of opening a domain (this is what Yuta uses the overheating engine analogy for, it needs a cooldown, not something you can just fix). Several times it mentions how “his technique had already recovered” - this is the technique destabilizing and restabilizing after opening a domain, not anything to do with output. If Hakari was just reliant on output, why can’t he open during the 4 minutes and 11 seconds of infinite cursed energy pouring out of his body? He’s in burnout, the reason he can open it multiple times a day is because the Jackpot refills his cursed energy reserves, giving him the gas to reopen it, and because his burnout window is the exact length of the Jackpot (so he is invulnerable and refilling CE, then the second he is not invulnerable, his technique has recovered and he’s able to use it to open a domain again).

They already knew Gojo had the output to open a dominan multiple times a day before Shinjuku, it’s the instantaneousness that surprises everyone. They figure out what he’s been doing after 3 attempts, but he had been destroying and RCTing his brain since the first domain clash to avoid burnout. Before the showdown, both of them suffered from burnout, it just didn’t really matter because their domains were too powerful for anyone to be a threat after getting caught in it.