(C_MountJournal.GetMountInfo(index) does not exist anymore since 7.0.3 I thought?)
Index is not the same as mount ID anyway, so you can't check spell ID of cast to see which mount was used in this way.
Either way, even if this method worked, it would just tell you "you just mounted onto a (flying) mount", which doesn't really help since OP wants to check for when you actually start flying to zoom out the camera. That being said, doing it at mounting time may be the best available solution.
If you're fine with just zooming out as soon as you mount then
this would be of interest to you. Otherwise DynamicCam handles this already, but I don't know if you can disable everything else and leave just the mount zoom out.