No, what I'm saying is that 7 or 8 top-end, young d-men isn't anywhere close to 1/3 of the NHL. I think you're saying that's almost 1/3 of the NHL's #1 defensemen, rather than just defensemen in-general.
Problem is that there are a minimum of 6 defensemen in the NHL per team. So we just named about 8 top-end defensemen, and there's about 180 or so defensemen to pull from).
4% of d-men in the NHL then (based on the 7 or 8 we said, can double the number if you'd like) would be considered "top, young d-men" among the NHL's players. So yes, among all defensemen, those 8 or so that were said are in fact rare.