Advising the opposite to humans does not make sense. 13 words is already tiny to convince someone. The choices I was thinking were restricting LLM word count and increasing human word count. The goal is specifically to make them more comparable.
The given study does not show any strength of humans over LLMs. Both goal metrics (truthful and deceptive) are better for LLMs than humans. If you are misinterpreting my advice as general advice for people not under the study's conditions, I would want to see the results of the proposed rerun before suggesting that.
However, if length of text is legitimately convincing regardless of content, I don't know why humans should avoid using that. If LLMs end up more convincing to humans than other humans simply because humans are too prideful to make their arguments longer, that seems like the worst possible future.