What Zeke opened up was the possibility for Eren to motivate Grisha to kill the Reiss, which Eren knew would happen from having seen Grisha's memories. There's nothing here that suggests the future-seeing ability is necessarily an effect of the timeloop, rather than the other way around. Speaking generically, simply being able to see future memories doesn't necessitate the existence of a timeloop.
I think people also forget though, Eren can see these memories also because he was connected to the royal, Historia. Which is why we could ask "why didn't Grisha or Owl utilize this ability?" It's not like Eren can just come and go as he pleases. So while Eren was able to manipulate Grisha, Grisha wasn't able to do the same to Eren, because he lack the connection to the Royals at that time.
He also manipulates Zeke only when Zeke allows him to peer through the past memories. It's the founding that has access to all the Titan's memories, it's the Attack power that can manipulates them.
If anything, it's the 1st King's memory that we haven't see, on why he made that renouncing vow etc, and what did he do with all that power in hand. Hell, we don't even know hoe the Ackermans came to be yet.
Isayama keeps on delivering, every chapter is a suspense, every chapter adds new clue, every chapter moves it forward.