Here contract of carriage https://www.united.com/web/en-US/content/contract-of-carriag...
We can read the definitions section of the contract where it defines oversold
>Oversold Flight means a flight where there are more Passengers holding valid confirmed Tickets that check-in for the flight within the prescribed check-in time than there are available seats.
So does this apply to this case - yes, even though this individual was removed to accommodate UA employees the fact was there were more passengers holding valid tickets than there were available seats
Now scroll down to Rule 25 which gives the provisions for what would happen in the case of oversold flights - one of which allows for involuntarily denying passengers
>Boarding Priorities - If a flight is Oversold, no one may be denied boarding against his/her will until UA or other carrier personnel first ask for volunteers who will give up their reservations willingly in exchange for compensation as determined by UA. If there are not enough volunteers, other Passengers may be denied boarding involuntarily in accordance with UA’s boarding priority
The article may have one valid argument - that the language of the contract says "boarding" not "leaving once already seated" but I don't know how much of a difference that would make in legal terms. Is there really a world of difference between not being let on the plane and being asked to leave once you've taken your seat. I don't think there is.
It's possible that you might get this in front of a sympathetic jury and get an award. But as a lawyer, under the law it's pretty clear that United did not breach any contract. And This remains true no matter what talking heads on TV are saying.