The debate is whether the air pocket is water vapor or not. My opinion is that is mostly air, not water vapor. His counter argument was based on the assumption that the volume of the air pocket increased at least 2x which cannot be accounted for entirely from expansion of air.
So the key is to find out whether the air pocket really increased by at least 2x. JFF seems to believe that you can just eyeball the volume increase. That's nonsense. There are many factors that can fool one's eyes.