That is the branch for development for using AIDL telephony interfaces with additional changes in libgbinder and libgbinder-radio. AIDL support for the non-VoLTE cases is generic and can be used on any devices which need it. Newer Android versions are moving to using AIDL for all hardware features while older versions used HIDL for those. Sony devices still use HIDL for telephony interfaces but at some point their future devices they will also move to AIDL because Android requires that now for new hardware platforms.
As for VoLTE in the C2 device, it uses a hardware vendor specific interface (unisoc) similarly as Sony and other Qualcomm devices use a different vendor specific interface (qti) so the VoLTE support currently being implemented will be useful only for unisoc devices. For Qualcomm device the already existing qti VoLTE-plugin is still used as before.