PWM motors can control their rotation very precisely unlike IC motors. That is one reason you see multicopter designs nowadays. You can quickly alter the torque. I didn't see how this design alters pitch or roll, but I know there were some coaxial experimental designs that can alter the torque in a single rotation to affect those axis. That is simply not possible with something powered by burning fuel. It might be possible to have a mixed system, using an internal combustion engine for primary lift and electronically controlled steering, but that would be a more complex system and the benefit isn't abundantly clear.