Waaaayyyy back in Ten Forward, Guinan should have said right then and there "Oh yes, this is Borg space. The Borg are a race of organic and artificial life. There are thousands of them. They are relentless. You cannot defeat them, you cannot reason with them. They will destroy you. Avoid them at all costs."
Because really... she said all that anyway. Only not up front. Why wouldn't she??
Aside from dramatic purposes that would have cut the eps short, if Guinan knew about the Borg, why would she NOT have revealed it all as soon as Picard asked at the beginning?
Perhaps piss poor relevant strategic communication was the reason the El-Aurians got their asses wiped out by the Borg in the first place