We’ll almost certainly see models trained to produce watermarks directly in the next few months. A highly diffuse statistical abnormality would be nearly impossible to detect or remove.
A robust rotationally invariant repeating-but-not-obviously micro watermark would be quite a neat trick.
Quantization & subsampling do a hell of a job at getting "unwanted" information gone. If a human cannot perceive the watermark, then processes that aggressively approach this threshold of perception will potentially succeed at removing it.
Because our main interest is this meaningful content, it will be harder to scrub from the image.
It seems much more likely that it's their solution to detect and filter AI images from being used in their training corpus - kind of a latent "robots.txt".