Well that is true in one sense. U21 NT manager is the one that makes the team and creates the features of that team by telling what to train etc. When it comes to NT managers, although he can still tell everybody to train, many managers might not, since the players are old and may not think that it's worth it.
So generally speaking, NT managers only have limited choices and players, whereas U-21 NT manager have more freedom in a sense he can make the team the way he likes it to be.