FOC current control autotuning PI Controller for BLDC Motors
It's quite possible they used the simplified model you cite.  It's how I do motor controllers.  It's based on the observation that the attainable bandwidth of the current loop is well over 10 times that of the attainable bandwidth of the motor speed or position loops.  That means that you can pretty much treat the motor dynamics as an independent DC disturbance to the current loop.

Now, should you be designing some super high-bandwidth motor servo loop and you find that the motor's armature control bandwidth is getting within 10x of the current loop bandwidth -- you'll need to change your thinking.