As others said, to be clear, no optimized-based training was involved ("To do this, I used OpenAI GPT-3 Playground. There's a maximum limit of how much content you can put in, so I selected snippets from around 40 entries from the ages of 7 to 18, but I focused on the ages between 10 and 14."). IE journal text bits were just put in the prompt as examples and then the author presumably told GPT-3 to talk to her as if she were her younger self.

To me it seems like entering a bunch of small snippets from a diary would result in a pretty generic impersonation of talking to your younger self, and that's kind of what the quoted bits read like. It's kind of disappointing, would be cool to see actually fine-tuned GPT-3 do this.