Pretty clear it's all AI. The @zigbook account only has 1 activity prior to publishing this repo, and that's an issue where they mention "ai has made me too lazy": https://github.com/microsoft/vscode/issues/272725

After reading the first five chapters, I'm leaning this way. Not because of a specific phrase, but because the pacing is way off. It's really strange to start with symbol exporting, then moving to while loops, then moving to slices. It just feels like a strange order. The "how it works" and "key insights" also feel like a GPT summarization. Maybe that's just a writing tic, but the combination of correct grammar with bad pacing isn't something I feel like a human writer has. Either you have neither (due to lack of practice), or both (because when you do a lot of writing you also pick up at least some ability to pace). Could be wrong though.