Theft means A steal something from B. It has nothing to do with B gives something to A voluntarily.
Even if a bank gives you $3m by accident, it's not a theft, especially if you don't touch it. But if you steal something, even if you pay it entirely back, you are in violation of law and face penalties.
So there is a huge difference and calling this a theft is wrong.
Finally in this case, it's impossible for the airline to claim that it lost money.