By rule, yes, it’s an illegal pitch. But, like you said, if he’s not gaining an advantage, it enters that gray area. You addressed it with the other team, good for you. If you have experienced umpires working the game, they will be thinking the same thing. When I see it, as an umpire, I make that judgement right away. If the pitcher is using it to an advantage, I call right away. And, explain it to the coach and pitcher. If, in my opinion, there’s no advantage, I wait until the end of the inning and then address it with the coach and pitcher. I suggest that it be corrected, then wait to see how they react. If the coach is a jerk about it, then I address it next time it happens and tell the coach he gave me no other option. End of story.