>I wonder if the rapid growth of ML recently has diluted the reviewer pool dramatically?
There was a blow up a couple months ago around this issue. Someone posted to r/machinelearning saying they were starting grad school in the fall, and had been accepted as a reviewer for NIPS2018.
Twitter convo about it [0] generated some press, with the pull quote of "It's 'peer review', not 'person who did 5 TensorFlow tutorials review'".