Going by Star Fleet's information, the Borg would assume Talos IV was an uninhabited planet with some sort of lethal space plague. There would be no reason to look at it, and every reason to avoid it.
General Order 7 doesn't reveal anything about what was there. It seems unlikely to be people. If they were primitive but exploitable, the Prime Directive already protects them. If they were dangerous, how? They don't have space flight. But for any sort of race, Star Fleet would engage with them in some way. Anthropologists, negotiators ... something. If they were a dying race of super-telepaths which could only be saved by human volunteers, Star Fleet would have kept trying to help them. A ban would have been ridiculous in that case.
The order is basically a quarantine. The planet has something the Federation is terrified of, which is contagious (otherwise people could just go there and die. No harm done to anyone else, no need for the rule). It might not be a threat to the Borg, but could be, and the Borg aren't all that interested in research anyway. The Omega Particle is an exception, but Star Fleet handles those incidents differently.
Whatever Talos IV has, the humans did the hard work learning to avoid it, with the Borg as beneficiaries of that knowledge. Assimilation works.