I agree with A&C here. On a low cost carrier, with a no refund policy, a seat sold is money in the bank. It doesn't matter how many pax occupy the aeroplane or how many seats depart with nobody sitting in them, the flight is paid for.
For your information, there are refundable fares (usually designated as FLEXI-something) even on low-cost carriers, so in that sense even a fully booked flight is not necessarily 100% paid for, unless everybody really shows up. Anyway, we know this was not the case on A&C's flight so it's not really relevant.