You're missing the point, I can name some coaches who have done well with a younger team. There are plenty of coaches that take a younger team and develop them into really good NBA teams. Flip had 4 really good NBA teams. Those teams were the MV3 year (all veterans, most of them traded for or through FA) and the Detroit teams (all inherited veterans)
Teams of all rookies might NOT start out good but there are plenty of coaches who have developed young teams into being really good teams.
Flip's really good teams have been built via free agency/trade or he inherited the players.
In his defense, he has never had chance to develop a lot of young talent, outside of his stint in Washington.