I have been feeding NLS for approx. 15 years, scores of fish, including many wild caught fish. Other than a wild caught juvenile Mbu puffer, which typically don't eat any type of pellet food, I don't believe that I have ever had a fish that I have not been able to train to eat NLS pellets. I have even trained juvenile Flowerhorn imported from Thailand, that were raised on an exclusive diet of fresh blood worms, to eat NLS pellets. For most fish, in 10 days or less.
If you can't train a freshwater fish to eat pellets, use the search feature and my ID. Your LFS owner is most likely enjoying a better mark up on other products, hence the reason he would state such a ridiculous thing.