You are right that some 4WD systems (generally not AWD systems) can have an advantage of engine braking in wet or snowy conditions by not having to use brakes as much. Most drivers overcompensate on braking application in snow or a loss of control situation, which is why ABS works so well against the natural human response. But if you have a driver that is aware of the vehicle mass and the road conditions a proper braking system wouldn't lose out to control of a vehicle being stopped to engine braking in a 4WD system. Most people don't have the driving skill to know when engine braking would even be advantageous over wheel braking. And engine braking done too aggressively is hard to quickly disengage comparative to letting up on too much wheel brake application which can lead to longer periods of loss of control.
Ultimately what Porsche lays claim to is outlandish in extreme situations. They can't beat the laws of physics. And unless their car has active tire studding it wouldn't stand a chance against a quarter mile stretch of wet ice at 4 degree decline. Doesn't matter what vehicle, tires or software you've got. That vehicle is sliding to the bottom with no control.
The reason is that there was no ABS, and the brake proportioning valve would send most of the stopping power to the front wheels, since most of your stopping power comes from them on the dry roads. But in snow, the fronts lock up before the rears are slowing you down much. With 4WD active, the front brakes were connected to the back as well as the front and would use their stopping power as well.
Today is a different world though, with ABS.
You're right, it's all about the tires. I used to drive an Audi S4 which I would put Blizzaks on in the winter. I would take people out in the snow and blast around, it was amazingly predictable and controllable.