if it is too small too see, then it's not sand, it's dust. that would not clog a pump so bad to cause it to break. are they essentially saying that their skimmers won't work in a tank that has a sandbed, because if so then they need to get out of this industry. The majority of tanks have some sort of sand bed. How would sand be in the water column anyway? Is you powerhead pointed right at it?
What they are saying is the equivilent of telling someone with a faulty door that if they want a replacement they better make sure the wind doesn't ever hit the door, because even an imperceptible breeze will cause it to break open...a poor excuse for poor craftsmanship.