If it's Borg, they're going to have to go into knots explaining why Jurati's Borg can't appear or help. She's the only friendly Borg they know and she might even be able to just pull the "evil" or whatever away from Jack.
Also, overwhelming, ancient evil darkness was not really the Borg's theme. They pursue perfection in their own twisted way and are ruthless and don't care who they assimilate to achieve it, but in their own misguided way they think they're doing good. As Locutus himself said, "We wish to raise the quality of life, for all species". It's far more likely that any origin of the Borg was a well intended science experiment gone horribly wrong, not an ancient race of evil beings like Sauron from Middle Earth or something.
Making the villain the Borg would be a very, very big tonal change from their past depiction to the ancient mystical evil vibes we've been getting around Jack this season.
I think ancient and weak points to whoever created the Borg or their progenitor species.