except second-hand accounts from the El-Aurians (who may not even have known where the Borg come from or where they vanished to).
The El-Aurians who survived might not have known at the time who it was who was destroying them, Guinan could have come by that information in later years through listening to others stories.
She put the pieces together.
Actually, you're quoting me
, not GameOn
But yeah, it's possible that it wasn't until sometime after the destruction of their world that Guinan and Soran learned the Borg were responsible for it (being from a race of listeners scattered across the Galaxy, El-Aurians may know many things that the Federation has yet to learn).