Correct, but misleading. The power is proportional to the square of the amplitude. Also, the people being in phase depends on where you're standing. If you stand in just the right (or rather, wrong) spot, you get 100 times the amplitude and 10,000 times the power, but if you average the power output in all directions, it would end up being 100 times the power.
If it's worth saying, but not worth its own post, even in Discussion, it goes here.