It's a game of cat and mouse in the sense that you can build LLMs specifically optimized for evading the current crop of detectors, but in my testing, they work pretty darn well in the general case. While they might not reliably pick on all LLM text, and while there's sometimes a couple of words in human-generated writing that causes them to output a low but non-zero probability of LLM content, they do not rate human-generated text as "99% AI". Especially not across multiple writing samples.
The most likely story here, I suspect, is that the person leaned on LLMs for commissioned writing and is now trying to save face. The secrecy of the models works both ways, right? And frankly - how often do you see people in HN, or people who do commissioned writing, admit in private that they're using ChatGPT? It's cropping up all over the place.
Note that I'm not commenting on the ethics, fairness, or transparency of tools like that. I'm just saying they work far better than you might be suspecting.