Am I misremembering, or is the first scene where Seven mentions the Borg incursion in VOY played as if the other characters have no idea what she's talking about? Could it be that Seven has got 'race memories' of the Borg incursion timeline, which none of the Starfleet personel recognise because they're all still part of an older order of things, pre-timeline change?
Voyager got sent to the Delta Quadrant in 2371. First Contact
happened in 2373 (SD 50893.5). They got Seven right at the beginning of 2374 (SD 51003.7 is "Scorpion Pt.2"). So they wouldn't know about that whole incident (and no one historically knew about the Borg, even with that one episode of ENT), but Seven would know because she was still a Borg when it happened.