The interesting tidbit here is SynthID. While a good first step, it doesn't solve the problem of AI generated content NOT having any kind of watermark. So we can prove that something WITH the ID is AI generated but we can't prove that something without one ISN'T AI generated. Like it would be nice if all photo and video generated by the big players would have some kind of standardized identifier on them - but now you…
Hell, it might even be possible for some arbitrary photographs to come up with an AI prompt that produces them or something similar enough to be indistinguishable to the human eye, opening up the possibility of "proving" something is fake even when it was actually real.
What you want just can't work, not even from a theoretical or practical standpoint, let alone the other concerns mentioned in this thread.