Well Zverev didn't even play that well. Novak just basically tanked the match after NOT converting the breakpoint in set 2. He won 1 game after that.Djokovic being 34 might have something to do with it. Recovery time and endurance becomes more of an issue with age.
Plus Zverev has smashed him before in BO3. It's not that big of a surprise. Zed's peak level is enough to beat any of the big 3. He just lacks the mental fortitude and consistency a lot of the time.
Honestly, Zverev didn't play anywhere close to his peak.