If you have some free time ( ;) ) I would be very interested in seeing a proper ranking. Maybe they're limiting themselves to "Deep Learning AI" when they say Montreal has one of the biggest concentrations of researchers in the world.
I guess you are right regarding Deep Learning. Even then, it might be hard to beat the Bay Area. Unfortunately it would take a lot of work to do a proper ranking (and many companies might not make the info public). Maybe we need a better AI for that. ;)
If you strictly looked at published papers on the subject matter and looked at the location of the authors you could do this relatively simple.