What he means is that according to the graph if you call yourself an "independent thinker" then you can't be an extremist. You are automatically a centrist. All the dots on the "independent thinker" half are all centrist. None are left or right. An interesting bias that he's admitting to. Made me roll my eyes and stop reading right there, and just skim the rest for all the "independent thinker" tropes.

If you want to feel superior and virtue signal, just label yourself an "independent thinker." It's so easy.