The block diagram on page 130 of the Firmware Reference Guide shows how this works.
The MixRack has the Mix Engine and does all the processing, except for surface IO which has its own preamps and ADC/DAC.
A MixRack will work headlessly, passing audio according to its last state, and can be controlled by an iPad or Surface(s) or the installed remote controllers which I know less about.
We have our Dante card connected to our MixRack so that even if we lose the surface, the audio still passes.
I have no idea why the surface talkback mic would still work if you lost connection entirely. If the cable was unplugged, it would not have worked, so you must have still had some audio connection between them, but perhaps a loss of control only?