I don't really think you did anything wrong, but I can see how some parents would especially at 12u.
I think you do bring up a good point, too. If this player wants to be a starting IF, then they need to put in the work to get better. Sounds like they are relying on your practices when that's not going to be enough.
Yup. Hard conversation to have maybe, but it's time to talk to AC and advise him that other players are practicing more than his DD and thus are better players.
I give certain 9 year olds on our team drills to do at home. I tell some parents that they need to practice outside of our scheduled practices if they want to improve and earn time at other positions. It's hard for me to imagine that this AC of yours doesn't know this, but even if he does, it's good for him to hear.