That doesn't mean that it can't be distguishable by some other means.
LLMs clearly pass the single sentence test. If you generate far more text than their window, I'm pretty sure they'd clearly fail as they start getting repetitive or losing track of what they've written. In between, it varies depending on how much text you get to look at. A single paragraph is pretty darned hard. A full essay starts becoming something I'm more confident in my assessment.
It's also worth reminding people that LLMs are more than just "ChatGPT in its standard form". As a human trying to do bot detection sometimes, I've noticed some tells in ChatGPT's "standard voice" which almost everyone is still using, but once people graduate from "Write a blog post about $TOPIC related to $LANGUAGE" to "Write a blog post about $TOPIC related to $LANGUAGE in the style of Ernest Hemmingway" in their prompts it's going to become very difficult to tell by style alone.