[bmacho]

I also made the same quick calculation, opening the door ~20-30 times is equivalent to placing a bottle of milk in it, in terms of joules. (Milk has ~4x the mass, and ~6x the heat capacity in terms of mass.) Is that negligible? Well, depends on the usage, i guess, but the 10x efficiency from the article certainly can't come from it.

Maybe if you open the door, the moving air heats the things inside much faster?

edit: in a sister thread they state that the humidity of air plays a more significant role.