I find myself wondering why people assume that the virus/puzzle brought up in "I, Borg" would have actually done...well, anything...to the Borg. Because Geordi said so?
Not the best argument, to my mind.
We also don't know that it would not have worked either. Given that they were planning to deploy it as a weapon they must have had some confidence that it would have worked.
We do know that in one potential timeline the Federation did create some sort of virus that is very effective against the Borg within 20 years of Nemesis.