Rubbish, the function of a phase locked loop is to keep the phase of the 2 signals the same, within the constraints of the loop filter.
The clock *never* achieves this, it is open loop and applies a 'kick' to one pendulum the amplitude of which is NOT related to the difference in phase of the 2 pendulums.
A fixed kick is given without any knowledge that it will be of the correct amplitude to achieve an in phase or near in phase condition. There is NO feedback of an error signal that relates to the phase difference between the 2 pendulums.
The only time phase comes into the picture is the timing of when the 'kick' is given, so as not to disrupt the normal swing of the pendulum, and whether or not to give a kick at all.
It is and ingenious system, but not a phase locked loop.
I guess it could be closer to a PLL if the kick had its amplitude varied by the phase difference between the 2 pendulums, but you still have the problem that if you were in the state where no kick was required there is no way of slowing the second pendulum without waiting for it to drift back, so it is still open loop.