Buffet is no stranger to aviation, and he's still holding a lot of exposure to the industry with Netjets, but there is only so much risk that a money manager can take. The risk for the airline stocks isn't that they just go down considerably and he loses money - if that was only the case he'd hold on like many other money-losing stocks he holds for the long term.
The risk here is that common stock is subordinated debt in the event of a bankruptcy filing, which has a higher likelihood as the coronavirus shutdown causes longer-term disruption than originally forecast. If that happens, your stock never bounces back and it's written off after bankruptcy filing, as even the bondholders will be lucky to be paid at that point.