Diana was more of less a beautiful memory to most of us until Harry started using her memory as a human shield and excuse for all his own bad behaviour. Including using the Bashir interview in his Netflix doc, that William and Charles Spencer had explicitly asked not to be shown again, due to the nefarious means employed by Bashir in obtaining it and the way he fueled her paranoia - that resulted in her getting rid of her palace security detail.
William was "parentified" by his mother (this is well documented) and unfairly used by her as a shoulder to cry on. Of all people, he would be well aware of her flaws intimately. Nobody is telling him anything he didn't know about her from first hand experience. She was his mum and he no doubt loves her unconditionally while still accepting her flaws.
What William's probably more unhappy about, more than an honest discourse of his mother, is his extremely dishonest brother using her memory at every given opportunity, in order to receive reflected glory and sympathy for himself. Because it is Harry's actions and constant name checking of his mother that has re-opened the book on her - the beautiful memory is being re-assessed thanks to her youngest son.
As someone who decided to leave London during the Diana floodgates, because it became impossible to get any work done so I decided to take leave instead, it's actually good to remember that history has been re-written quite significantly post her death. She was not exactly vilified before it, but neither was she universally beloved, and hardly anyone I know with a modicum of sense believed in the Saint Diana fairytale, either before or afterwards.