The linked study does show that correlation. But I personally believe it’s more of a second-order effect.

Vegetarianism selects for openness and analytical/reflective thinking but the extra step to veganism selects for absolutism/purity and rigid category-based thinking. Reflective thinking and openness are definitely correlated to IQ so I believe the result in the article.

The point here is to know your audience. The people on this site are perfectly primed to respond well to a vegetarian/environmental point of view.