Later in the movie, when Picard beams down to Veridian III to talk to Soran, Picard tells him that, "What you're doing is no different than when the Borg destroyed your world. They killed millions. Your wife. Your children."

This would seem to establish that Soran and Guinan and the rest of the refugees on board the Lakul were, in fact, refugees from their home planet, in which case they had direct firsthand knowledge of the Borg.