Your question would be answered if they would call it driving assist and not auto pilot. Then you'd understand that you would not turn it on in snow. Just like Cruise control.

If my 2015 S 70D can't see enough to tell that there is a lane then it won't allow you to turn on autosteer. Generally if the road is covered in snow then it won't turn on, also it will turn off if the lane markings disappear.