I would say a large garbage bag would be as warm as that vest
and would provide better coverage from the elements. A tyvek
suit, especially the coated models would be better yet.

The air mattress comparison is a great one.

Micro convection larger voids in still air occurs whether sealed off or not in larger voids. Why do you put fiberglass in your walls, or special gas in thermo's.

This vest is stopping heat loss by convection, evaporation, helps in the short term with stopping some wetting of the clothes underneath by rain or snow and so is useful, but the air pockets are kinda gimmicky without some method of stopping internal convection.

I would take a trash bag over that vest given the choice, even if they were the same price, due to the better coverage of the garbage
bag. You can make a small tent from a garbage bag and even use a heat source under the hem as the cavers do. I carry one in the lining of my climbing helmet.