(1) The importance of this type of contest is a central point of David Donoho's 2017 article "50 years of data science".

(2) Was the choice of criterion "RMSE" rather than "predicting exact rating (1-5) correctly" a stroke of genius, a stroke of idiocy, or mere thoughtlessness? Because the "kitchen sink" outcome, that for RMSE it is optimal to average over many equally good algorithms, was certainly predictable. The alternative criterion might have produced a winning algorithm that was both more implementable and more human-interpretable.