Yet another article is wrong... this wasn't overselling, and it wasn't bumping. Overselling and all the regulations about it are about boarding. They don't apply after the passenger is already boarded. This was strictly about whether United had the right to do "denial of transport" which is different.
It's true that overbooking and bumping happens a lot, but it's false that this was an instance of that.
It's true that overbooking and bumping happens a lot, but it's false that this was an instance of that.