1. yeah Luffy lost, that’s literally my point. He came back to life because he’s pirate Jesus. You cannot nor should expect a villain to have the same plot powers as the MC.
2. it’s simple, the flashback makes it clear that Kaido believes Joyboy is the only man who would be able to beat him and King easily dismissed it. That would not make any sense if King was there to witness all of his loses. Meaning Kaido clearly lost all those times before he became a pirate captain. At this point Kaido losing is a big deal, and he will likely not get back up for a second fight, again that is MC things, not villains. When villains get knocked down, 99% of the time they’re out for good.
3. it was him hyping up Kaido off manga. I’m old enough to have gone through a few author comments in other series to know better than to take ‘em seriously. It’s literally just advertising