The fact is unless these models are not what they promise to be, there's no way of weeding out generated content from not without meaningful false positives.

On the other hand, if the models are in some way deterministic, then it stands to reason there's some plausible method of detection.

But if that's the case then there's a tremendous flaw in the foundation of these techniques...they aren't at all what's been promised.

So far that doesn't seem to be the case.

Finally it's worth pointing out that Google et al really shouldn't care if content was generated by a machine or not. The problem isn't content creation, it's content quality. So what Google actually cares about is delivering a result that meets a certain quality threshold. It's very difficult to believe Google cares much about the process of content creation.