I think this might be because rather than 'write how you talk', what you actually want to do is write how you would talk if you had time to say the best version

I think this might be because rather than 'write how you talk', what you actually want to do is write how you would talk if you had time to say the best version of your thoughts. This means if you couldn't see yourself sitting down and saying the stuff you're writing to somebody, it's probably no good. If what you write doesn't have a natural flow when spoken from the tongue, it probably won't when people read it either.