The weirdest thing about this episode was when Guinan and Q were about to duke it out and Guinan's pose looks like she's going to claw Q's eyes out.
That's just a cute visual reference to an 'iconic' pose she makes in the movie "The Color Purple"
. And the reason she's so vague about the Borg is just to make them seem more ominous with her dialogue. I think we're overanalyzing a bit here.
If she actually gave them a detailed, logical rundown about the Borg right off the bat, it wouldn't get across the basic message of 'the Borg are supreme badasses and the crew is in some deep shit now' quite as powerfully as, "all I can tell you is you better start heading back RIGHT NOW"