Based on UAL's Contract of Carriage, they violated their own rules. This flight wasn't "OVERSOLD," because the four employees were NON-REV. They didn't buy a ticket, thus they weren't "sold" anything!
Oh, those pesky little rules.
Plus, as I mentioned above, the removed pax had already boarded, so by definition his boarding couldn't be denied, as it already took place.
Rule 25 Denied Boarding Compensation
Denied Boarding (U.S.A./Canadian Flight Origin) - When there is an Oversold UA flight that originates in the U.S.A. or Canada, the following provisions apply:...