That's interesting!
Very nice idea (and very nice implementation!).
In my opinion an angle sensor can be enough (it certainly is enough in Matlab simulations).... and it is certainly less expensive than two sensors (BTW, I'm saying that because cheapness was one of the aim of the project).
The robot works pretty well even with only a gyroscope... if you start your own project keep in mind that an angle sensor (or maybe gyro + accelerometer) is better.