No one is going to claim RWD is better than AWD in the snow.
What they will claim is that RWD with good snow tires is better than AWD with all seasons.
If you had an issue then you'd have to ask if you were stuck on a patch of ice and if anything would have given you that pickup. The Pirellis are also a performance snow tire, so if you live in a very snowy area then you should have gotten Blizzaks or a true studless snow tire.
Current: '15 M235i Vert
Previous: '13 335i, '08 TL-S, '00 Corvette