I can see the validity here if what he's saying is right - if other themes don't have a dedicated design team (but just one team that does several themes at once), but castle has a dedicated design team that only does castle, that is indeed good news.
It means LEGO is dedicating a larger number of resources to the theme (people and money-wise).
This increases the probability of a nicer product in the future because when people only focus on one thing they get more involved (instead of multitasking on many different things), and it also indicates to us that castle is probably making money and not going away anytime soon (I don't want to go back to the years of only HP and SW sets!)
So while I don't know where you found this out, or if it is correct, I would like it to be true!