In the fiction you can be so much more truthful without worrying all the time about causing direct pain. That's another book. I think this one tells a considerable amount about how somebody, in this case me, becomes a writer. In the end, there is someone who comes along, another voice, that questions, not the truthfulness, but the ability of the writer to be revealing in this form. This isn't sufficiently savage.
Candor's a kind of cover.
Now one shouldn't accept what Zuckerman says at face value. And he also makes a good case as to why he's a better vehicle. The autobiography consists in part in the clash of those points of view, of being torn between the facts and the fiction, torn between the autobiographical impulse to understand something and the fictionalizing impulse to understand something. Zuckerman casts doubt on the project, and this doubt, this uncertainty about my perception, is a powerful part of this writer's makeup. You're not sure you have a purchase on things.