I Can Only Assume That You Are Correct...
The true test is to call an actual Mercedes dealer's parts department. I would assume that there is something in the stuff that is not designed to be mixed with standard windshield washer fluid, and that what they are saying is that you should only mix with their winterized product, which is likely to contain a windshield de-icer.
"We drive into the future using only our rearview mirror."
- Marshall McLuhan -
Northern California Wine Country...
"Turbos whistle, grapes wine..."