Do you want to work on Project Soli at ATAP? Which part, the actual hardware / Radio antenna design? Then a PhD in analog design might be required to be qualified. Do you want to do the signal interpretation / analysis to detect gestures? Maybe a strong background in signals, filtering or even machine learning may be required.
HCI work can involve a lot of specific specialization which often requires these research / industry giants to demand a strong testament of your qualifications. Although not required, a PhD is often an easy way for them to see that.
If it is the more 'high-level' stuff, I'm not sure, as sometime a 'PhD in HCI' itself may be required (although I don't know too much what a 'PhD in HCI' entails)
Source: Experiences with HCI PhD's at UC Berkeley