It's not irrational to think that launching an ICBM might involve a computer and therefore a mouse. Hell, I'd be surprised if it didn't. I doubt the Pentagon has posted videos of whatever it is they use on YouTube, so it's silly to ask for proof.

As I said, I agree that it is irrational to think the guy was whiteboarding an ICBM launch. (Not to mention while terrorists could conceivably get a dirty bomb, an ICBM seems well out of the question.) But there is nothing in those photos that would lead an average person to believe it was a game.