I got that tip from David Chang and he wasn't specific (re the grit). My guess is
would the grit drop off if you agitated them? Like tossing them (dried) in a salad spinner or a colander? I'd give that a try. Otherwise, just pulverize them in a Cuisinart.
I heard of this trick several years ago from another woman who hated the texture of mushrooms. She loved the flavor, but not the texture. So she pulverized dried mushrooms into a powder and used it on pizzas, pastas, soups, etc.