Earlier quoted context omitted.
Does that count only the actual airport? It doesn't count the potential business travellers contributing to the economy in different ways. Like if half of the financial sector were due to arrive at Heathrow, where would that be in the analysis? Just to downplay the importance of Heathrow through numbers is a bit absurd.
If half the financial sector is in the plane somehow, and they decide “oh, the airport has a power outage, better crash into the ocean instead,” that might make a major difference. More likely they will go land at a different airport or delay their travel, depending on there they are in the process. It’s just delays, not destruction.
You missed the parent's point, which is that a significant fraction of people flying to London because they have business to do in London, and the value of their time is not zero. If their time is wasted, that has a real cost in terms of lost productivity.