The article didn’t say that, did it? I do wonder why jumbo wasn’t told to take the next flight since she was late, tho.
It doesn't say she was late. It just says the other woman sat down first. Airlines often start giving standby fliers seats before everyone is on the plane and seated if they think they have empty seats. The teen could have also been stowing luggage or talking to someone they were traveling with that was sitting elsewhere on the plane. They might have even moved the teen from where she had been sitting to the seat next to the one that wasn't booked in advance so she would have more room.
When you fly standby, the airline accommodates you if they can. Passengers that booked in advance get accommodated first.
If it is the first time the teen has flown, or has flown since gaining significant weight, she can claim ignorance, and unless she was traveling alone booting her off causes additional problems. Next time she can't and should be forced to book two seats.
I'm sure the airline would have preferred to sell the additional seat. They didn't do this to make more money. They just did what they could in a bad situation.