I typed this from memory, so probably not repeating LLMs word for word. If I change that prompt to "As an AI language model..." the detection works as expected. In a sense, a broader point my original comment demonstrates is that even for an absolutely obvious AI-generated text, auto-detection can't work reliably because it's trained on specific responses of specific LLMs that can be altered at any time.

To be clear: not attempting to discourage you. It's a very complex and interesting problem to tackle.