Times have changed a lot since Red's coaching days. As well as the pro sports players psyche. I think Red wouldn't have much of a problem bashing his players in the press if he was coaching nowadays. Except Russell, of course; I don't see Russell as someone who would handle this kind of stuff in a nice way...
This is a very mundane thing to say, but players are individuals and they react in different ways to different approaches. Some guys respond well to harsh criticism, others not so much.
I see Doc as a very good coach all-around, but he has a history of excelling when it comes to motivating players.
Of course that when you are primarily focusing on developing and showcasing youngsters, you aren't properly coaching to win. Some may call it tanking, and I'm okay with that. What shouldn't be overlooked, I think, is that it is extremely difficult to bring your guys to play every night (a very underrated skill in a team), to bring the effort every game, when you are coaching under those conditions and most of your players are mediocre/average. Doc has coached some very untalented teams, especially the one in the year before the last, but they always showed up consistently and played every night. Without that, Ainge wouldn't be able to make the trades that put the C's on the top.
p.s. - oh, answering the OP question, probably Doc was simply stating the truth. Scal is the 2nd best defensive player in our bench so I don't see this as surprising.