The reason I could think for going external is no hardware change if the circuitry isn't there on the boards now to measure the motors power -- and we know they use (at least) an external tach reader. And so instead it might be easier, cheaper to provide an "input pin" and some software changes while using existing electronics with an external source of info on the load.
But someone said they put a load on the back of the tender itself (not the coupler was my interpretation) -- and this invoked the behavior... so maybe it is the motor control approach.
Or maybe that's the mock up for the show (proof of concept), and the product in development will differ... etc...
As I said, DCC seems to definitely have sound keyed to load or some boards do it & they use just motor control.
