It might be just as crackpot as many other theories I've seen floated here and there, and I'm not even sure it fits how technology works in Trek, but if Maddox or even Soong used Borg technology to create androids/synths, then perhaps the very act of making Borg and Federation technology compatible with each other could open a gigantic backdoor for the Borg to be able to access and subvert every piece of networked Federation technology. Imagine if the Borg were able to use that backdoor to reprogram every single replicator to saturate every meal with nanoprobes. Realistically thinking, it can be anything at this point, and the only clues we have are the Zhat Vash being ancient, Soji being called the Destroyer, Maddox sending her to a Borg cube which suffered a submatrix collapse after assimilating a couple of Romulans (which reminds me a bit of Icheb as a genetically engineered anti-Borg weapon), and the fact that Oh/Jurati thought Maddox was too dangerous to be left alive (not to mention his lab being razed by the Tal Shiar to destroy all his research). So I think the most probable reason is that Maddox's synths were based on Borg technology, and there's some connection between the Romulans/Vulcans and the Borg. I am eager to learn more in the coming 5 episodes because I'm finding it a good mystery so far.