I agree with this.
We know BJ has a reputation for lying, so I don’t think anyone expected him to tell the truth in his memoir. As you say, I think people read the memoir partly because he’d been PM and held other high political positions and also because he’s a good writer - he’s funny and entertaining, and can tell a good story.
With TSP, however, I’d argue the main selling point was precisely the fact it was marketed as being ‘unflinchingly honest’ and that it described these awful events that had happened to the Winns/Walkers with that level of honesty.
For me, the difference between BJ’s memoir and TSP is the differing USPs of entertainment value v honesty.
Reflecting on this, I think Alexei Navalny’s memoir is a good comparison to TSP in terms of USP, as both have a common hook of incredible events happening to their authors.
For anyone who doesn’t know, Navalny was famous for being an ordinary citizen who stood up against Putin and who was then poisoned on Putin’s orders, imprisoned in Russia and then killed (the circumstances of his death are still unclear).
Navalny wrote a memoir about his life: this covered his life before becoming an opposition figure to Putin and included sections about his poisoning and his imprisonment in Russia (via diary entries and letters he sent from prison).
We know he was poisoned, we know he was imprisoned and we know he died, as we have clear evidence of all of these things. I think all three of those things give his memoir an unflinching honesty which, alongside Navalny’s fame, helped to popularise his memoir. It also helps, of course, that his writing is funny and entertaining. However, I don’t think his memoir would have been so popular if he hadn’t gone through those things I mention here.