It's not a continuity error.
Having bits of knowledge about a potential enemy is a wholly different animal than having them on your doorstep and dealing with them on an ongoing basis.
I tend to think Starfleet swept the events of Regeneration under the rug. The last thing a fledgling space exploration program needs is the knowledge of cybernetic vampires becoming public.
And don't forget that the events of First Contact/Regeneration/Dark Frontier are a direct result of Q introducing the Borg in Q, Who?. Sometimes 'cause and effect' can seem like 'effect and cause' in the Trek universe.
Agreed.
But what most likely happened is that SF actually had limited data on who the cybermen were... at least in the days of Zefram Cochrane an Archer.
This limited info wouldn't really give SF much to go on.
At least, not until the late 23rd century when the Al-Urians fled to the Federation.
SF started piecing together a THEORY on what the Borg could be, but since the only 'real' encounter they had was the one from 'Regeneration' and of what Cochrane said (remember that they never took him seriously due to excessive drinking), it was limited.
The Hansens were able to piece more data together before leaving due to their unconventional approach, and while it might have been a SF mission at first, they broke away from their ties pretty fast.
Even in the 22nd century, boomers were aware of species that SF captains and their respective crews weren't.
I would surmise that since limited info on the cybermen was gathered, the data in question and the 'Borg' were officially a theory, it was either not publicly available, limited to selected personnel, or just plain available to everybody with no one bothering to look it up or was able to piece it together.
So it's not really surprising that no one on the Enterprise-D (apart from Guinan) knew of the Borg.
Besides, the very nature of the collective and just how powerful they were would still remain a mystery to SF by the events of Q Who.
So Q's introduction of the Borg wars not really diminished because no one on the Enterprise-D was able to piece those together.
Besides, they would probably start piecing info together after the initial introduction, or until events of FC happened/passed.
Also, it's possible that Picard or someone on the
Enterprise-E during FC events studied data that SF had available, pieced it all together and figured it would be best to leave pieces of the destroyed Sphere on Earth.