What I've always interpreted this advice as as is to read out loud what I write and then eliminate the parts that sound weird or like a robot.
Source: Writing for more than half my life and HN has liked some of my articles in the past
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.
This is how I write
I literally use my iphone voice to text to transcribe, then go back and edit.
It’s surprising that more people aren’t doing it
My current best way to write is to just write everything that amuses me, then go back and edit out the tangents, and then usually reverse the order of the paragraphs.