How do you figure Zack is so much stronger than Cloud? Sephiroth beat him badly, and the only reason he wasn't out of it like Cloud during the trip to Midgar was because the Jenova cells didn't have any effect on him. You could argue that they had no effect because Zack is so strong, but then why did Sephiroth, who was affected, beat him like he did?

Furthermore, Cloud wasn't running on adrenaline, at least not entirely. We went through most of the game thinking of Sephiroth as some unbeatable god, and that flashback served to finally make the revelation that Sephiroth was in fact a wimp compared to Cloud.