The stuff about overbooking is not relevant here. This flight was not overbooked. The carrier decided to send staff members by it although all seats were sold.
Overbooking normally works for a variety of reasons, first is that departing a hub (as here) with many/most having connected, a proportion typically do not make it because their inbound was late. Another significant proportion is those high-payers with flexible tickets change plans on the day, in particular meeting ended early and they transfer forward to an earlier flight, freeing up seats on later departures.