The implication was that what was seen as Irumodic Syndrome was a planned event and either "a long game" by the Borg or early infiltration by the Dominion.
Well, the wormhole was opened on stardate 46379.1. TNG ended on stardate 47988 (this is when Picard was basically diagnosed with I.S.)... that's about a year and a half difference.
We know the Dominion infiltrated the A.Q. in the first 2 years of the wormhole opening... they captured ships and people in the G.Q., studied their databases with technology, cultures, and had time to adjust their weapons to bypass Federation shields, etc.
Its possible the Dominion didn't start infiltrating the A.Q until just after the first 6 months or 1 year tops... this would certainly give the Changelings time to give Picard I.S.
As for the Borg premise... eh, its a possibility.... but we witnessed Jack having telepathic capabilities, and was able to transmit his movements to LaForge's daughter, along with his thoughts... something more akin to the Great Link and Borg consciousness (whatever that may be)... but how we was able to initiate such contact with another human who doesn't have the syndrome is puzzling.
Yes, we saw telepaths transmit their thoughts to humans in Trek (and on VOY, the Inarans were able to share memories and experiences) but on the whole, this was very rare.
Troi at one point mentioned that the Cairn couldn't read human thoughts and could only communicate with other telepaths because human brains weren't sophisticated enough... but we saw that other telepathic species CAN read human thoughts... so this seemed like the Cairn's telepathy not necessarily being strong enough to connect with (usually) non-telepathic species (though in Trek, humans seem to have the potential for that).
The Changelings did initiate a telepathic connection for the purpose of sharing information, experienced and to exist in the great Link... thing is they never demonstrated telepathic abilities with other species, nor did they engage in mind reading (otherwise they wouldn't need to spy on the Federation and other species to learn about them).