I hear and read this all the time... water droplets on leaves form lenses so direct light can burn spots under the droplets. If this is true then what happens when it rains and the bright sun emerges from behind the clouds? I've never seen any burn spots on leaves that weren't caused by chemicals or nutrients.
It's not just stoners... many other gardeners repeat the same thing. Sometimes a bit of nonsense is started and it grows into an urban myth that lasts for many decades. What vexes me is the complete lack of logic even when the evidence to the contrary is overwhelming.