In the actual study (linked to in the article) they mentioned testing this out with the coffee or tea powder substituting 1, 2 or 3% of the flour in the recipe. Also note that the used coffee grounds and tea leaves were repeatedly rinsed with distilled water, then dried and made into a powder. So, different than just used grounds or leaves that you would have handy.
Still, this might inspire me to find some existing recipes that call for coffee grounds and tea leaves.