Any AirTag owned by anybody can and will communicate with your iPhone over Bluetooth if you have the right features enabled (which are default). Your phone will contact Apple and tell them about the interaction. I haven’t seen protocol details, but last week a security researcher demonstrated that it is possible to use the AirTags protocol as a covert channel (with very low bandwidth).

I recently read up on Bluetooth low energy. The AirTags are most likely periodically broadcasting announcement packets, and folks' iPhones are passively receiving them. It therefore seems comparable to using 802.11 SSID beacons for geo-location, except that folk intentionally bought the AirTag to be position tracked.

Just by modulating whether or not the AirTag is transmitting, it seems like you could communicate at a low data rate. You could even use hundreds or thousands of AirTag IDs to get a useful data rate. Just because you could do it, though, it seems like there are plenty of other more practical covert channels.

not great, but not the same as open connectivity to unknown devices.

Amazon Sidewalk also is not "open connectivity".

No, it isn’t a great comparison. But it is a similar slippery slope.