Of course I'm not sure. it's his autobiography so he's free to embellish all he wants. Not like those girls are going to pop up to tell their sides of the story. He does talk about how getting with girls and then cheating on all of them and ruining all his relationships is a recurring theme in his life (and his dad did the same). Plus, considering he was a fancy-pants london-west-end-acting art student, he was exposed to lots of stuff far earlier than teens usually are (especially in the more sheltered 60s). So it's not that much of a stretch to believe.
He's pretty self-critical and self-aware though in regards to many things (there's a chapter later on called "Live Aid: My Part In It's Downfall" so it's not like he blindly thinks everything he does is awesome).
We'll see how he tells things later on when I get to the parts of his life I know more in depth already.
He does consider his solo music to be mostly good though. that's an embellishment that will come as a surprise to nobody. I'm kinda hoping he'll shit on songs written by the other genesis guys, that would be amusing. we'll see.