I agree that this could be a tactic of deflection, that fits with Harry's narrative of a leaky palace and camps briefing on each other through their aids; and fits with his own repeated claim that MM is not the source, and now the claim that people close to her don't want to speak to her either. I assume even King Charles is allowed to have a private life and private correspondence that he alone opens and decides who to share it with, so it wouldn't be hard to figure out who the culprit might be, if it's true. I wonder what the legal ramifications would be for this invasion of privacy. Ii's one thing if the letter writer and recipient is willingly allowing its contents to be leaked, even via a friend. Very different if the letter writer/recipient has had it leaked against his express permission and in invasion of his private life.
Here's the interview, the remarks about the letter are from 4.30. I had to watch it a couple of time to get through OS's word salad.