I scanned the post on pangram and got the same result. Also, I'm not sure about you, but using AI tools for AI detection seems more rigorous than people going based off counting how many em-dashes there are and similar vibes.

You shouldn't. You should instead take note of "it's not X, it's Y" and em-dashes.

It seems plausible that someone could make a decent discriminator for distinguishing human text from AI text. I wouldn't immediately discount the idea.