I know that's what it says, but I definitely use it on mine every single day without a problem.
Maybe there are "grades" of fine sand. Mine's pretty fine, I'd say, but possibly do some research to see if there are levels of coarseness when it comes to sand. Again, I use it every day on my 100% sand substrate and have never had a problem. It doesn't suck sand up, actually. Somehow, I don't know quite how, the mechanism is calibrated so rocks and sand barely move. (They're lifted a tiny bit, but nowhere near where they would be sucked into the machinery.) The only thing that is really scooped up is the waste, in my experience.