“One thing seems certain: write as simply and plainly as possible and it’s more likely you’ll be thought of as intelligent.”

A professor at Princeton reporting on his work about reader perception of the writers intelligence. I wonder is there any thing or effort that isn’t _as possible_
I take his advice to mean,

use phrases and words most often used by others,i.e., cliches.

I’d rewrite his comment this way (avoiding thus, so that you think I’m more intelligent)

One thing seems certain: write as simply and plainly as possible and it’s more likely you’ll be thought of as intelligent.