I think this is the right take. The game has very weak rewards and clearly by saying “if you wait you will get two marshmallows”, some number of kids will assume they want you to wait and do it for that reason rather then the desire for a second marshmallow.

As a side note, I have a four year old and have no idea if he even would eat the first marshmallow. Sometime I imagine he would eat it instantly and then he would talk about it for days to us (“can you believe they had a marshmallow on that plate!”). Other times it would remind him of birthday cake (or something else) and he’d just save the marshmallow in his pocket forever. For example, he still has half his Halloween candy and it’s late February.