How can you prove provenance if anyone can use it? The point of SynthID is that Apple makes the hardware and OS and can reliably insert the watermark/signature between when the camera takes the photo and when it becomes available to any 3rd party software.

Because they have this pathway only they can access, their key and signature can be trusted. I’m not gonna trust Joe Schmoe’s signature that “No I didn’t use AI” unless I already trust Joe Schmoe (and in which case, he doesn’t need a watermark, I’ll just believe him when he says it).