Wait, wasn't Seven being tested by Ducane on her knowledge of temporal whatevers
when she spoke of both First Contact timelines? In that case, she probably got the complete picture from the USS Relativity's computers. They'd trained her before sending her back in time.
Prior to "Relativity", Seven said only that "the Borg were present during those events" (referring to First Contact) in "Year of Hell" - something implying first-hand info (i.e. from the message sent by the "Regeneration" survivors) rather than some collective-wide Spacebook update posted by the queen seconds before plunging into the time vortex. After all, someone like Seven wouldn't say the Borg WERE present if the last she knew was a plan to maybe hopefully if successful perhaps alter history.