The reason is quite simple - the headunit only displays information (and sends some ibus telegrams based on the buttons you press), it is not in charge of any sound output/processing/whatever.
If you still think their is a "special DSP onboard monitor", you should check out ETK or realoem, to see there is no "DSP" and "non DSP" part numbers for the headunit (should be in the Audio, Navigation, Electronic Systems section).