I think anyone who's spent any amount of time interacting with LLMs would easily tell the human response apart because LLMs have a distinguishable style of writ

I think anyone who's spent any amount of time interacting with LLMs would easily tell the human response apart because LLMs have a distinguishable style of writing that's mostly consistent across interactions. Gemini, for example, loves bullet points and splits its responses into multiple distinct sections with subheadings. Furthermore, you'd have to limit the scope of questioning to topics that the human on the other side is familiar with, otherwise they won't be able to come up with an answer to questions that aren't common knowledge, while an LLM will always happily generate pages of plausible-sounding (even if incorrect) text. I think a more appropriate benchmark would be subject matter experts interviewing an LLM and another subject matter expert, with clear guidelines regarding the style of writing they're expected to match.