I think it is more of a function of contact patch, rather than ground pressure.

The metal studs make a small pit in the ice when it comes in contact with the ice, and the larger the contact patch there is, the more studs and rubber than can make contact with the road surface, which gives you better grip.

Besides, freezing rain is rain that freezes with anything it comes in contact with, as long as it is not on the road. There's a lot to be said about the efficiency of our hydrocarbon use inside cities, because it is always a few degrees warmer inside cities due to the amount of cars and homes giving off heat.