Dileo was (probably) the primary read, but he was not the only read. Gallon and Funchess are running their own routes on that play and we could have thrown to them as well. The problem was that Gardner was starting to feel pressure (Kalis didn't protect that well) and made sure to get rid of the ball a little earlier than necessary.
Too many people here are trying to fit that play into their own preconceived notions about our offense/playcalling. It's not easy to score from three yards out on one try.