I won't reiterate what has been exactly said, but workouts like pushups (generally dismissed as biceps workouts) work the abs, too. So don't discard these or even other types of "curls" (shoulder presses, Y presses, rotational Y presses, etc.) because they benefit your shoulder strength and in some cases your core (rotational Y presses).
So no, the biceps are not directly or solely beneficial to a pitcher's delivery (yes to the deceleration, but not the only muscle (correct me if I'm wrong but aren't the triceps active in it, too?)), but do not discard workouts just because they work the biceps.