I have corrected that. It was supposed to say "None of this document was written by AI."

Thank you for spotting the error.

Understood, thanks for updating it!

I thought this was an accidental double negative by the author -- trying to declare they wrote it themselves.

There are some signs it's written by possibly a non-native speaker.

I don’t think the author knows that double negatives in English in a sentence like this cancel, not reinforce, each other.