Mostly I'm warning against putting your trust in Mythbusters. If you have other reasons to disbelieve, that'#s fine. For what it's worth, you don't need a laser. Light from the sun is already as near parallel as makes no difference. The size of the mirror makes a difference. What the mirror is made from makes a difference. The number of mirrors makes a difference. The soldiers would have had more time for training, and more motivation, than Mythbuster's staff. I agree they might have used a better target than timber; the boat's sails, perhaps, or rope rigging.