Do Apple Airpods Support HSP?
If you look at the Bluetooth diagnostics in System Profiler on a Mac, you'll find that the AirPods are listed as offering the "Handsfree" service, not the "Hand-set" service. 

The full list of services supported are: Handsfree, Wireless iAP, AVRCP Controller, Audio Sink, AVRCP Target, AAP Server.

For the Pro model things are a bit different.