Ok, that's the step that was missing. I couldn't figure out how there was a benefit to the users without data being fed back and data can't be fed back without knowing some ID.

So, while Apple doesn't know the ID of the person sending the data, they have a 'room number' that links back to an ID.

If Apple were to decide to scan photos for pictures of 'lines of white powder' they couldn't tell the police your name but they could say that the 3rd party knows who you are.