Rich,
GM is likely stating a position based on the worst possible outcomes, and a position that shields them as much as possible from warranty expenses. I can't imagine the worst happening every time B20 is used by every LMM owner.

I'd not worry about it, but I'd also not use concentrations higher than B20. If you can avoid B20 during the cooler months of the year, I'd avoid it because of the longer and more frequent DPF regens.