To kind of expand on what Jason's saying a 3D pendulum can be completely defined using just (x, y, z) and you can deduce the angles from these coordinates. In your case the pendulum only has two degrees of freedom (x and y for instance and z be calculated because the pendulum has a fixed length) and is why Jason suggests using only two generalized coordinates. A quadcopter does actually have 6 degrees of freedom and is why it would use (x, y, z) in addition to (pitch, roll, yaw).
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