Retroactively, it is an easy fit to be convinced of.
Achamian offers reflection a few times on the striking resemblances between Celmomas in his Dreams and Kellhus, c. when they first meet. As noted, if the Mandate believe, preemptively, that "an Anasurimbor will return" and Kellhus knows that Celmomas dies in the Mandate founder's arms... Seemingly simple dream-communication with a second inutteral for 'across time' rather than 'across space.'
Though, it seems we could only be discussing this about a Bakker book. I mean, we're questioning the gods-damned narrative structure to posit bullshit and it makes sense.