But the goal is to include "Genius level" content in language understandable by persons with about fourth grade reading skills.

It's not clear what this site is evaluating, but the guess would be that a "Genius level" evaluation means "incomprehensible to almost everybody" - regardless of subject matter or significance - and hence is somewhat of an insult.

Some years ago, the mantra was that newspapers and popular books/magazines should NOT be written above some fictitious fourth grade level (vocabulary and structure), since that keeps it "easy to scan" for skilled(?) readers and still allows presenting some content for those who struggle. Since the elimination of meaninful content (and replacement by ad-speak) in most such places, it's not obvious what the new rules are.

Of course it's also true that "back then" fourth graders were taught sentence structure and "parts of speech," and that seems not to be common now.

John