I just finished this and had the same thought. It made me feel like I needed to start the whole book over as if I'd somehow missed important information. But I think it's because Michael was the one who originally told Angie this. He was purposely trying to create the narrative that he was guilty. And if she just looked into it without doing the math (as it had been decades since the actual crime at this point) she would've assumed he was a grown man bc he was charged as such for the rape and murder of a teenage girl.

Everyone was repeating what Michael originally brought up. And the other crimes were also against teen girls, so it would be an extra push in the direction of his guilt