It is identically zero at a zero price and has some higher magnitude than that at a higher price. This point is obvious, but not often made explicitly in the economics literature. In fact, I don't know of anyone who does put forth this myth. I don't see how this is relevant to the seminar at all. Industrial organization is complicated, so we may need more that one way of looking at issues.
Perfect competition is very rare outside of agriculture and financial markets.
One of the reasons that it suits the airlines to sell tourists and other infrequent travellers an advanced purchase economy seat at a low price is that a late-booking business person trying to get onto the same flight may find economy class full and so have to endure free booze and ample legroom in business class, at five times the fare that the tourist paid. This does not require that every sale, or even every flight, be profitable. I noted that this seems a good example of price discrimination. And just because not every flight is profitable doesn't mean that the airlines aren't seeking profits wherever they can find them. They often cross-subsidize.