Knowing Thomas Cook's business I would expect them to get maybe one or two no shows per flight, and quite often zero. To have 30 sounds like an aircraft type (or configuration) substitution.
I think some of the comments above are from those who are writing about mainstream scheduled services, where you can get a lot of no-shows (eg booked 75 over on a 747, still finally went out with a few empty seats). This is not that type of operation.