Hate to doublepost, but Delta is definitely not reporting these as IDB despite the obligation to do so if the flights were indeed oversold.
IDB statistics - somewhat out of date, but between Q4 2017 and Q1 2018, Delta reported precisely one passenger that was IDB but did not qualify for compensation due to getting there within 1 hour of originally scheduled arrival time. Unless Delta started doing this practice only in the past year, the number of corroborating stories on this thread implies that it is endemic.