In the ST:TNG episode, Yesterday's Enterprise, Data says that Guinan's species has a perception which goes beyond linear time. Why would Data in the alternate timeline think that?
Back in the main timeline, when Guinan was ripped from the Nexus in "Generations", part of her was still left there. She told Picard she couldn't go back with him because she was already there. The part of her left in the Nexus, a place where time had no meaning, gave her her sixth sense. This explanation was in the original Generations script but was later edited.
GUINAN : It took a long time, but eventually I learned to live with it. And I began to realize that my experience in the Nexus had changed me...I knew things about people...about events...about time...
PICARD : Your "sixth sense"... I've always wondered where it came from
From this, one could think her abilities are not a feature of her species, but of her personally. It seems the Nexus gave her and a handful of other El-Aurian refugees have the ability to perceive time differently.
In addition to the Generations movie, the novel, Engines of Destiny, also makes it clear that Guinan's abilities are due to the Nexus.
Yet, how did the Enterprise crew in the alternate timeline know? Was Guinan involved in some temporal exchange before they encountered the Enterprise-C? And that somehow in that prior event, they became aware of Guinan having an extraordinary power?
If they didn't know, they would never have trusted her advice to send the Enterprise-C crew back through the temporal rift. Please don't tell me that Picard was willing to send them back because one more ship would not have saved the Federation from surrender, but the sacrifice of a single ship long ago would have. He decided to send them back before Data mentioned that the Enterprise-C's sacrifice would convince them to negotiate with the Federation for peace. Picard never would have sacrificed 125 lives solely on intuition. He would have let them live out their days in the current timeline. Unless he knew from experience to trust her.
To be clear, I am not asking why Picard and the Enterprise-D crew sent the Enterprise-C back in time. I am only asking how the Enterprise-D learned of Guinan's abilities.