It really looks like Google is able to detect the difference between GPT3 and human content.
The reason for that is that even simple GPT-2 models are able to detect _some_ content as generated, but they are too weak.
[Originality.ai](https://cemper.co/originalityai)* says they can reliably detect GPT-3 content, and indeed some examples look much better.
But Google themselves have bigger LLMs than GPT3, like PalM(560B vs. 175B parameters) so it is very likely that they are even better at generation, and detection.