The point is, if United wants to kick already-boarded, prepaid, well-behaved passengers off for any reason that suits the airline, their ToS needs to state that—loud and clear.
The only ambiguity I see is the meaning of the word "boarded." It's in the airline's interest to fly the plane full, and sometimes business conditions favor flying airline personnel over paying passengers. They don't bump people for fun and jollies.
Either way, when somebody is involuntarily denied the flight they expected, the remedy is paltry. If you really have to be there, and it really really matters, and your business can't stand ANY possibility of being bumped, don't fly commercial. The other choice is to bribe the airline with lots of business and money, because that is how they set their passage priorities.