Was Holyfield beating Tyson really a massive upset?

Yes, however it really shouldn't have been. Everything people said about Holyfield looking shot is completely true but he was still fighting at a world class level. And had been the entire duration Tyson was in prison plus before he went away.

Tyson was fighting cab drivers. I don't even know the best guy he faced post-prison; it was either Bruno, who Tyson already stopped and plenty others did as well or Bruce Seldon, who wasn't all that good. Holyfield was a huge step up in class for Tyson and would have rightly been the favorite if not for his health issues. But it was most likely Holyfield cycling his PEDs IMO that gave him his "heart condition". You don't miraculously get cured of a bad ticker and carry on an athletic career for another 15 or so years.