It's been awhile since I read Countdown, but I think Narada was actually refitted with a Romulan-built knockoff of Borg technology, created from the study of the real deal. Not actual Borg tech.
And, as Sci pointed out, A Singular Destiny leaves the door open for that kind of thing anyway.
Likewise a while since I've read 'Countdown' - I recall actual borg tech being used on the Narada in that comic - no cheap knock-off.
As for - 'A singular destiny' - it established the only surviving borg tech was fully inactive and all attempts to activate it will turn it into ash.
If someone actually figures out how to reactivate borg tech - easy coming back card for the borg.
The Typhon Pact novels have certainly dropped hints toward a Countdown-ish future (Spock becoming a legal resident of Romulus in 2283, Picard offered the role of Federation Ambassador etc.) but it's not a sure thing yet.
Hopefully, they won't go all the way - Data's resurrection, Vulcan-Federation relationship making the Andor-Federation one look healthy, borg - really, these are not good creative decisions.