He may actually believe that. Lots of people in pool stores are honest people who have been "educated" by the pool chem& equip companies to believe a pile of barnyard slush. They aren't "crooks" because they've been trained that this is really what needs to be done. It's wrong. They don't necessarily know it. The corporate heads do, of course. Look at Leslie's campaign against high phosphates. They even off a phosphate remover "plan" of selling you the stuff on regular basis. Yet I only know of ONE member who has had an issue with phosphates, and HE got around it by raising his Borate level (using Borax) to 50ppm. So, despite have phsophate levels 6x higher than the level Leslie's says is "too high", he has no algae.
But the clerk doesn't know this. Maybe. Probably.

Carl