So much minimising on this thread. William's Harry's big brother - Harry probably looked up to him growing up. I think you can be an adult, 6', army trained and still feel fear from family when they are angry and not feel you able to fight back every time. You don't have to be physically weak to be assaulted.
I can't see why Harry would lie about something like this. If it really was the case that Harry can say anything and then RF can't reply, then you could make up anything to make yourself sound better or the story more colourful. The details don't sound fake - the details sound mundane and real to me.
My opinion of William is much poorer now. The excerpt doesn't sound like a verbal argument escalated into a physical one. Harry said to William that he wasn't helping and William took offense to that and punched him. That's pretty awful.
I'm not staying Harry is right to give away private details like this either, but I can understand why he wants to get his side of the story out there.