> This meant it became profitable (i.e., above the marginal costs of the flights) to fly passenger aircraft purely for the belly-hold freight.

That's not the usual definition of "profitable". As you say, they're turning a gross profit (if we treat aircraft leasing costs etc. as fixed, which over a short timeframe they are), but it's not enough to cover their fixed costs, so grandparent is correct that they are not (net) profitable.