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.