That's a very cool fridge. But how much difference does that make in practice?

Air doesn't have much mass, right? How much energy does it actually cost to cool the air in a fridge? (vs the solid parts of the fridge, and the food)

Looks like the OP's fridge uses 10-20x less power than a typical fridge, is that entirely due to the air not spilling out?

Depends a lot on humidity. Condensing a fridge full of humid air releases a fair bit of heat.

Also a fuller fridge will have much more thermal mass and care less.

But yes, exchanging the entire air volume of a fridge every time you open it is very energy-wasteful.

Mostly yes. Upright fridge and freezer designs trade off efficiency for convenience (rooting around in a chest fridge/freezer can be annoying). https://youtu.be/CGAhWgkKlHI