Well it means they have Serie A experience, that is an undeniable fact. So they obviously have an immediate advantage, in terms of the style, language, culture and all that comes with it. So it can't be said that it means nothing, that is not true.
Of course, it doesn't at all mean that they will then be able to play at a high level for a club as big as Juventus. Likewise, because a player does well elsewhere does not mean they will do well here, so it's useless making bold statements about how these young players would be so much better.
When you pay for youth you pay for talent and promise but should also expect naivety and inconsistency. The kicker is that there is no guarantee they will ever live up to that promise. If there was then SDs wouldn't go for safer moves.