I wonder how they controlled for the ideomotor effect - if you think about moving a muscle without moving it, you tend to still flex/move it slightly even without noticing it. This might be enough to explain the strength increae.
Some strength gains from strength training are a result of more neurons making connections from the motor center of the brain to the muscle tissue. Thinking about moving a muscle might make more neurons form to the muscle, making more of them contract when it does move.
