Good ideas write themselves
Having good ideas is most of writing well. If you know what you’re talking about, you can say it in the plainest words and you’ll be perceived as having a good style. With speaking it’s the opposite: having good ideas is an alarmingly small component of being a good speaker.
Graham’s perspective of writing seems to expect good ideas to be clearly articulated in writing. If the idea is fuzzy, good writing will not clarify it. However, if the idea is clear, the writing will appear good.