That they sent a fleet of Cubes to that one planet, as well as Guinan saying that they came in true force (implying she didn't consider that one Cube a force) in the FIRST Borg episode shows that there are different levels of Borg assault. Hell, even Q mentioned about "provoking the Borg!". It was also shown that the Borg can and DO ignore species entirely.
Releasing the virus WOULD have provoked them, and brought on a heavier assault.
How were those 100,000 people more a threat? They never said, but it was perfectly in keeping with Guinan's "When they come they come IN FORCE". Those 100,000 people merited a REAL response UNLIKE the Federation. Just because they give us their life history doesn't mean they weren't considered serious by the Borg.
Why didn't the send a fleet, when we've seen the Borg send fleets against others (like the 8472)? Because they don't rank as a threat by the Borg. Simple logic.
Until you give proof that the Borg see the Feds as a worthwhile threat and that the Virus WOULD have worked beyond "Because Picard and co thought so!" (which means nothing), you're the one doing the runaround.