I keep reading "muscle memory" but the theory that one pilot shut down the engines instead of performing another action has nothing to do with muscle memory.

Muscle memory allows you to perform both actions effectively but doesn't make you confuse them. Especially when the corresponding sequence of callouts and actions is practiced and repeated over and over.

All of us have muscle memory for activating the left blinker in our car and pulling the handbrake, but has anyone pulled the handbrake when they wanted to signal left?