I think a lot of things but I also don't think it would serve to help William to leak that type of thing either.
Not to mention that the British press have a few things that are absolutely off limits but health is one of the sacred cows that they observe unless it's spoken about by that person.
The danger for William would be to damage the monarchy and for Harry to retaliate anyway.
So no that argument doesn't make sense.
And if it were to come from elsewhere that says a lot about the extent to which Harry has burnt bridges elsewhere with his behaviour.
For me that makes it an acknowledgment of weakness not strength in his feud with the Royals.
As it is he's not learnt the lesson of the Streisand effect and why the royals have the 'never explain never complain' strategy in the first place.
Nor the one about never interfere with your enemy whilst they are making their own mistakes
For me the whole issue here is Harry making bad decisions and never having faced up to consequences over the years because he was overprotected. It's suddenly coming to a bit of a head where there is a dawning realisation he's not as protected / he's unaware of how protected he's been and it's catching up with him.
Even the Nazi uniform thing hasn't had the same consequences it would have had for mere mortals.
He was undoubtedly warned by numerous people in his life to not do drugs etc etc but he did them anyway and continued...