How exactly does Tyson beat Ali...?

He got right back up proving he wasn't close to being knocked out. Ali didn't get knocked down because he was hurt, he got knocked down because it was 15 rounds of war and Frazier finally got him with a good one, and he got right back up. He didn't get hurt that much even after all that.
Tyson wouldn't have been able to do that after 15 assuming he'd be able to go 15 with a Prime Ali anyway. Ali's ability to move his head and body in ways that would allow him to absorb or avoid the full force of blows if not evade them completely was unparalleled.
When he was getting hit, there were so many times he took damage that should have put him away, and it never did.