Most flights are return trips, so that 6bn seats corresponds to 3bn potential unique people. Then there's the percentage of multi-leg trips. (For example, on my most recent Athens-Istanbul-Stockholm return trip, I alone took 6 seats total).

Finally, we have to consider the percentage of flights taken by people who fly more than once a year, which probably represents a majority of seats.

All in all, it probably still lines up with 10%. It's shockingly high to me as well…