Quite the opposite, as this text ignores the fact that Rule 21 includes unforeseeable conditions as a reason to refuse of transport. Here there was an unforeseeable condition, the need to use seats for a replacement crew because UA4600 Denver-Louisville was going to be delayed for an unknown or very long time, perhaps even cancelled, making it impossible for the UA4600 crew to fly the next morning out of Louisville as scheduled.
Also it is very naive from that lawyer to think that when a passenger's butt touches a seat they must ride no matter what, just like when a NFL player is down the very millisecond his knee touches the grass. UA might even have decided to turn the plane back after departure to pick up the replacement crew if the UA4600 delay had been known after departure, if they calculated that the costs for the extra op, fuel and IDB compensation would have been less than the costs of disrupting an entire day of flights for the aircraft and its passengers.
He doesn't sound naive in the least. He sounds like he is saying that for all intents and purposes his client was boarded. Since he was boarded United had no authority whatsoever to kick him off. There is a list of reasons why United can kick him off. None of those boxes seem to have been ticked.