Ah ... but that is distinct from someone being so bumped who is from the general public.
Will they toss such a person off, presumably without their consent and some consideration as compensation?
I don't know but I think they would. I actually was a member of the public because I didn't work for the airline. I was surprised they let me travel on such a ticket but they did.
According to the tweet, the person in question was a United pilot. He claimed she was "stand by", but pilots call it "deadheading".
https://en.wikipedia.org/wiki/Deadheading_(employee)
They will only fly if the seat is empty, and there's no paying customer to fill the seat.
Will they toss such a person off, presumably without their consent and some consideration as compensation?
Airlines overbook some flights, and they ask for volunteers first. They are usually offered a confirmed seat on the next flight, overnight accommodations if necessary, and a voucher that can be used for a future flight.
If they don't get volunteers, they raise the offer until someone accepts it. But at some point, they may bump off people involuntarily. The Dept. of Transportation has rules for compensating people bumped involuntarily:
https://www.transportation.gov/airconsumer/fly-rights#Overbooking