You're claiming the argument is wrong, then supporting it again.

The person(s) who go over or under the threshold are the ones that make a difference. If thresholds aren't breached by the amount of people changing route, then there is not a very significant difference.

As you indeed state:

"but you can still very much say the climate footprint of each passenger is (total plane print/passengers)"

You can indeed say as much, but "this is oversimplified".

Just to be clear - I am not arguing that we should not pick routes responsibly, with the environment in mind. I am merely trying to ensure we are clearer about what impacts we are actually making and that we are considering all the impacts and not just doing a simple calculation that actually misses out the many complexities of transport.