First issue is "Private until you test positive" is incorrect.
The broadcast identifiers roll with the BT MAC addresses, assuming your device is sane enough to do that rolling, so you can't go from a diagnosis key to a specific device. The diagnosis keys also don't provide a mechanism that allows you to determine the device - going forward or back from the published ones. The protocol does not include any identifying information - the only thing that can associated day keys with a specific device is the diagnosis key server, and that doesn't have access to actual contact info, so can't build up anything from that.
Secondly there's the permanent tracking infrastructure: this API and the associated crypto by design do not include sufficient info for tracking.
I think the problem here is that people keep reading the Google simplification (that imo is slightly misleading), instead of the actual protocol and cryptographic specifications provided by Apple. The latter of which explicitly spells out the privacy compromises from a person who publishes their diagnosis keys.
