It's definitely not a downmix, you can clearly hear some sounds in the mono mix which are not in the stereo mix.

For example, when Luke is playing with his Skyhopper wingtip then suddenly puts it on the table next to him and says: "C'est vraiment pas juste. Tous mes copains sont déjà pilotes, moi je suis prisonnier ici!" You can hear the sound of the wingtip hitting the table in the mono mix but not in the stereo mix.

I've checked a lot of different scenes, these are two different mixes.