As we don't know what's been said in the interview, nobody can say that Charles speaks any untruths about Diana, their relationship or anything or anybody else.
And seven, eight years after her death, and now that he is married to the woman he really loves and his sons are doing well, Charles has no real need to say anything negative about Diana. She isn't alive to counter anything he has to say, so he has everything (or at least popularity points) to say positive things about her. And he may actually mean these things he says about his ex-wife! (Imagine that!)
Well certainly their father's own words about his life, their lives, or Camilla can't be worst than what the tabloids say about William and Harry on a weekly basis.
I would think that St. James' Palace would not let Charles do something stupid on such a public scale. And in that sense, they would be protecting the Queen and the monarchy.