I think there’s a difference between saying your STBXH had been having an affair, and talking at length an in detail about the whole history of the marriage and what you thought of his family.
Diana was v loyal to the institution and the family. Her criticisms of Charles were not a ‘tell all’ - there wasn’t much beyond the fact of the affair (which was hugely well known at that point). She wasn’t particularly specific, and the only other criticism was that she, personally, thought that the ‘top job’ would not be a good fit for him. Said to undermine him? Yes. But she made it sound like she was speaking from concern.
She never did any form of exposé of what went on behind closed doors. Harry has gone much further than either of his parents ever did about his family.
And even if you think my thoughts on this are wrong, and that what Diana said did amount to an exposé (and that case could be made depending on what you think the extent of her cooperation in the Morton book was), I think this is the territory of two wrongs not making a right.
The Sussex children are already going to find out loads of stuff most of us never know about our parents. And Ai just don’t see how a loving, child-centred parent would want to add to that be doing a public hatchet job on their father