It makes sense, because if you think about it, the MCU could read the sonars value, to insure that on both ends its the same as when it was first calibrated (0), and as it increases on either the front or back one, the motors could accelerate in the opposite direction, to insure that it stays afloat, and the encoders would insure the velocity is proportional to the distance needed to travel to stay upright. Many balancing bots have been made using only encoders, sonar, or IR detectors, it really is all in the code.
Side Note: how the hell can that cost $25,000 to make?!?!? Is that yen or some Korean currency??