That was after two savage beatings by Tyson, though. Especially the second one must have left its mark.
Holyfield and Bowe both avoided Ruddock. If they'd been forced to face him I'm sure both would take the offer, but Lennox was the one who stepped up when the others didn't. When Tyson went to prison, Ruddock was the WBC #1 contender and next in line to face Holyfield after thrashing Phil Jackson.
Hard to say for sure, but it's not unlikely that reflexes and chin both are effected by beatings like those.
Tyson might have a better chin than Mercer, if not the same level. He is constantly coming forward and that amplifies the effect of the punch. Holyfield dropped Mercer with a single left hook while Mercer was on the back foot. Had Mercer not taken a knee he might've been KO'd. Tyson took flush hook after flush hook and got stopped on his feet. I'd say they're on the same level. Oliver McCall takes the cake for me.
You think taking a knee in somehow lessens his chin? Tyson was that hurt in the first Holyfield bout he was completely out on his feet and didn't know where he was.