They are providing this tool for users to run. Eventually someone will just use this tool en mass, then use this data to create a local model that will detect stuff with Google's watermark.

> They are providing this tool for users to run

Where? From what I see in https://deepmind.google/technologies/synthid/#:~:text=Where%... , it is not a tool, it is an "undescribed algorithm". Otherwise people will immediately:

1) build an algorithm which slightly modifies few pixels to break SynthID.

2) apply SynthID to real images to imitate tampering made "allegedly" by specific people