I still love PAD's connection of the Borg and the Planet Killer from Vendetta, though I think Destiny's new Borg origin might kill that.
Eh. They thought the Planet Killer was older than it actually was. Maybe it was designed to give off false energy readings for some reason we don't yet know. Maybe when they were both stuck in the Galactic Barrier, they were caught in some temporal loop or some such and so aged many thousands of years more than the rest of the universe did before escaping.
It's fudgable.