Not sure it would have needed to have been 10%. How many Concorde regulars were there? There can't have been that many!

Assuming two flights a day (is that right, before the AF crash?) at two-thirds occupancy, that's 130 people each day, or only 650 pax each week in each direction. Suppose 100 of those were regular passengers - I would be surprised if it was more than that.

40% of that is only 40 people.

The company in question lost 658 employees in the attacks! It's perfectly possible that a couple of dozen of these - not necessarily 40, of course; other firms would also have been represented - were NYC-LON commuters on Concorde. It was a business that made nine-figure profits with only 1000 employees, so you wouldn't expect them to travel at the back of the bus.