I always understood it as advice for school children who don't know how to express themselves in writing and end up with a blank page. You are encouraged to imagine you're talking to someone to overcome the hurdle of translating thoughts into words.

Along similar lines, in elementary school we were taught the introduction/body/conclusion pattern to help organize our thoughts in essays, but actually using those words as headers makes you sound like a child. "Conclusion" is especially common. It just reads like you're blindly following that pattern and don't actually know how to tie your thoughts together.

I remember editing my then girlfriend’s (now wife) papers in university, when she was struggling to explain something succinctly. I’d ask her: well what are you trying to say? And she would explain it to me, clear as day. “Just write that.”