They shouldn't be operating below costs at any time. That is anticompetitive and disingenuous.
In the US anti-trust is absurdly focused on whether there is costs in the form of high prices passed onto the consumer.
They didn't say unprofitable companies shouldn't exist, but there's certainly a case to be made that large companies swooping into a market, reducing their prices to squash competition and then raising prices is not good for consumers, and there's governments that make this practice illegal. Intent matters.