Zack is alive in another timeline. There are a lot of people saying that it's just a "flashback" and Zack dies shortly after from his wounds, but this just isn't the case. They show the glitters or whatever falling and healing Biggs who wakes up (Timeline change, he was supposed to die), you then see Zack get back up after the explosion, he is now healed and able to carry Cloud.
It's now a split timeline. Like I speculated a few pages back, this is all about merging timelines and getting that perfect happy ending. Aerith will live, and I would not be surprised if part 2 that we play as Zack at some point, At some point, maybe at the very end, the timelines merge, Zack reunites with Aerith, Cloud and Tifa get together, Jessie stays a dead thot - fan fics everywhere come true.