They do have friends though, who claim to know all the inside, and hitherto unrevealed, info of what happened when they exited the RF. There's a close friend of MM who did an interview on This Morning the day after there Oprah interview, and said MM has the receipts for everything she said on Oprah. She also was very clear that H&M knew she was doing the interview, she wasn't speaking against their wishes. Scobie doesn't need to go directly to H&M if their friends who claim to be in the know are given the green light to say what they know(including Eugenie, who was name checked in the extract from the book in People Magazine - a reference to a conversation you'd think only she and Harry would know about). H&M can plausibly deny it was them who spoke to him.
I disagree that Harry is above petty gossip, though. didn't he include some stuff about his dad and his teddy bear in Spare? Completely unnecessary. And he hates Camilla, so I don't think he cares about her feelings at all. It's funny how everyone forgets that "Tampongate" was an early incidence of phone hacking.