I agree with you. That is the literal definition of an agonist.
Still, in casual parlance we often use the term ‘agonist’ to denote an agent which potentiates or stimulates the receptor regardless of its mode of action. An example might be any of the acetylcholinesterase inhibitors (i.e. donezepil et al) which while not directly stimulating the acetylcholine receptor, have a net effect to augment cholinergic transmission.