With decent processing a mm-wave picture would be good enough that you could recognise a person, so I don't think there is any inherent anonymity. The image resolution is a function of both wavelength and aperture and a mm-wave antenna can extend over a significant number of wavelengths (ie. large aperture). A stable timebase would further allow processing across time, enabling synthetic aperture.

Also, a person is not a randomly shaped object. If the processing is specifically tuned to detecting and identifying people an awful lot of degrees of freedom can be eliminated, giving more detail in those features that do identify a person.