Just because Luffy is also fatigued, it doesn't make this a 1v1.
Think about it.
Do you believe Luffy could do what Kaido is doing right now?
Fight the 9 scabbards, including Inu and Neko in sulong form that destroyed Jack, fight the roof 5, fight yamato, K.O a bunch of them, and not get any breaks?
defeat Kaido 3 times during all of this?
It's nuts to say that Luffy is the same as Kaido here.
The whole in universe world views it as a 1vs1, and Oda views it as a 1vs1 autistic forum posters be damned.
that is the reason he has directly set up Kaido as "the one to bet on 1vs1" and had Yamato worry about Luffy saying "no one can beat him 1vs1", "are you sure you want a 1vs1 etc." and has had Luffy explicitly tell MULTIPLE people to get the fuck out of the fight so he can have a 1vs1.
Kaido has MYTHICAL AWAKENED ZOAN REGEN AND STAMINA AND IS NEARLY IMPERVIOUS TO DAMAGE beneath the threshold that Luffy is currently wielding. Kaido hasn't been knocked out, so
relative to his own hp bar he has taken less damage than Luffy has, who has been knocked out twice.
That's like saying can you expect Kaido to tank and regenerate damage better than Shanks? yes. Does that mean it is inconceivable for Shanks to beat Kaido 1vs1, no.