In the ICE world the advantage of FWD cars in snow was related to the mass distribution. More mass over the front wheels produced better traction. Without an engine in the front compartment, and the heavy battery pack on the floor, I'd expect a Model S to be better than a standard ICE RWD vehicle because the mass would be more over the rear wheels. Moreover, the torque of a motor can change much more rapidly than that of an engine, so traction control will be superior in an EV. Finally, because the battery pack in an electric will increase the mass, the traction will be better to begin with than what you'd find in a comparable ICE vehicle. So several advantages to the BEV. (Plus no need to heft sand bags into the back!)
Given all these factors, it wouldn't be surprising if a RWD BEV performed as well in snow as a AWD ICE vehicle. Assuming of course similar tires. Those make a big difference. On ice I'd take anything with studded snow tires over anything without.