I have one, and it doesn't leak any light (and I would be very suspicious of it if it did). I would suspect the ads of being photoshopped, myself.

I leave mine on 24/7, but then I'm using it primarily for pathological microorganism control than algae control. I've recently read something that implied UV could help with water chemistry, but I didn't understand much of that.
I do bear in mind that it's, like, the cheapest UV filter on the market, so I'm not expecting the world out of it. Of course, the light canister being sealed means we do kind of just have to trust that it's working... unless somebody has some spare fiber optic cable lying around. XD