AirTags contain lithium primary batteries, which are a totally different thing (other than both containing the element lithium).
Lithium primary batteries present no more a risk than alkaline AAs. Probably less.
Lithium poses two risks:
1) The internal resistance is low enough that if it's shorted it can go into thermal runaway. This is the risk they had in mind when saying no loose cells (but note that cells merely need to be securely contained, not specifically in a device.)
2) Secondary cells can grow whiskers inside the cell. If a whisker grows just wrong it can short the cell from inside and drive it into runaway. This is the risk that they are worried about here--and it's a legitimate risk, it's brought a plane down.
The reason the rules are different in the passenger compartment is that while there's nothing on board an airplane that can fight a lithium fire it's generally a small, weak fire (the big e-bike batteries that have been in some rather dramatic videos aren't allowed) that humans can generally keep from turning into a big fire. But if nobody can fight the fire you have a big problem. Hence why lost phones are treated very differently--they could have fallen someplace where the fire wouldn't be fought.
How does that tell you that they are dangerous? The danger with rechargeable Lithium batteries isn't that they contact water and begin to burn.