This seems a bit different than what you suggest. The paper shows the "average rally length" and "# of hits per minute" increasing as time goes on for the group

This seems a bit different than what you suggest. The paper shows the "average rally length" and "# of hits per minute" increasing as time goes on for the group which received additional stimulus as feedback when it hit the ball. It did not find an increase in these metrics for groups where the input signal went silent as feedback after hitting the ball, nor for groups which merely continued receiving the usual input ("no feedback"). I have no idea if this is repeatable, but it seems like a valid conclusion from this data that the neuron colonies which were given feedback in the form of extra stimulation, exhibited better performance after the first 5 minutes of each trial, and better performance each subsequent day when they would run another 20 minute trial.