They didn’t overbook. They just sold tickets for a scheduled departure for which they couldn’t seat all the paying passengers.
"On April 9, 2017, United Express Flight 3411, operated by Republic, was preparing to depart Chicago O’Hare (ORD) to Louisville (SDF). Republic Airline made the decision to assign four of their crewmembers to deadhead on Flight 3411 within minutes of the scheduled departure."
To this point, I've been willing to presume ignorance and confusion on the part of FReepers who have gotten it wrong. The presumption is about to shift.