No, there's no mistake, the back EMF from the motor winding traveling through a magnetic field has nothing to do directly with the current going through the motor, although it does interact with it, what you may be thinking of is the inductive EMF that's created from the motor winding inductance, this is separate from the EMF that the coils generate because they're traveling through a magnetic field.
Take a look at the first part of the spice model the electrical part, that's pretty basic the inductance of the coil is there, the motor's voltage source is there as well as the resistance, there should probably be a parasitic capacitance added but this is not going to seriously effect anything. There's a component at the end called VSENSE which is actually a voltage source but it's value is determined by the second mechanical components circuit which multiplies in the Vsense value and feedbacks a multiplier which is the torque constant produced by the motor in line with the inductance and a resistor which account for inertia and friction and speed on the mechanical simulation side.
I'd love to play around with this a bit as all you have to do is plug in the right numbers and you can actually simulate the effects of specific terrain types. If you develop it enough you could add in tire spring and traction constants for gravity and corning, there's really no limit to how far you can take this kind of simulation if you want to spend the time on it.
Spring however is rapidly approaching and aside from the idle time I spend here on the forums which is generally when I'm resting my brain actually working that kind of thing out has no appeal to me unless I have an immediate practical need for it, which as I'm busy as a bever my hobby skills grow very slowly.