VW ran some tests on biodiesel and found that high levels of fuel dilution didn't cause significant wear to the engine or problems with DPF longevity. In fact bio was found to tax the DPF less. In anycase the issiu with bio is a lack of standards. IIRC the EPA doesn't have specific quality standards. My guess is that BMW was thinking worst case (backyard bio) when deciding on an acceptable concentration.
The VW articles were in a Lubricant Industry periodical.