I've never heard "acorn-beneath-the-cup mentality", but I imagine the reference is to the game - or rather confidence trick - described in this Wiki article:
The shell game (also known as Thimblerig, Three shells and a pea, the old army game) is portrayed as a gamblinggame, but in reality, when a wager for money is made, it is a confidence trick used to perpetrate fraud.
The game requires three shells (thimbles, walnut shells, bottle caps, plastic cups, and even match boxes have been used), and a small, soft round ball, about the size of a pea, and often referred to as such. It can be played on almost any flat surface, but on the streets it is often seen played on a mat lying on the ground, or on a cardboard box. The person perpetrating the swindle (called the thimblerigger,operator, or shell man) begins the game by placing the pea under one of the shells, then quickly shuffles the shells around.