There are many mechanisms for crew (and just airline employees in general - think mechanics on an emergency call-out to a remote airfield) to pre-empt paying passengers.

This case was exceptional in that those crew (presumably) showed up to the gate after boarding, but I've watched a set of mechanics walk up to a gate just as it was boarding with "must fly" company tickets and some of the last folks who had yet to board got denied.

I don't think there is a significant difference in calling this flight overbooked due to the 4 extra seats needed were on company tickets - it just typically happens behind the scenes 99% of the time and is invisible unless you're specifically watching for it.

In the age of social media 'must fly' company tickets are likely to become a thing of the past. As is offering minimum compensation and if not accepted forcibly 'volunteering' a couple of passengers. Spectacles like this are the last thing any public facing company can get away with and if they got away with it in the past it was only because there weren't any easy avenues for people to hold them accountable.

United seems to be on a roll lately.