Payton had him on the team. He had a chance to evaluate him and then let him leave. I'm not saying it was a mistake, but it is an example of a player that Payton let leave that went on to have a good career somewhere else.
I don't really see it as a mistake when that happens, because there's no way of knowing how the player would have done if the coach kept him. It's possible that a player doesn't play as well had the team kept him as he did after going to another team.