People
I never think at all when I write. Nobody can do two things at the same time and do them both well.
— Don Marquis

Behind the line May contain spoilers.
Our reading
An editorial interpretation.
The speaker separates thinking from writing, then uses a general claim about doing two things badly at once to justify the separation. The joke makes an absence of thought sound like disciplined concentration. Its neat logic becomes comic because the two activities are treated as unrelated tasks.
Context reviewed