The linked article and video never said that it wouldn't work on non-human moving objects, just that it would work on moving humans (as it is a wireless relative-motion detector)

It is possible that this tech is especially effective at tracking humans due to the fact that we're bags of mostly water and the specific frequency used interacts with water more than other nearby frequencies. (I'm not a microwave engineer, so I don't know this for sure)

You are correct. We can determine what an object is made out of or at least behaves like based on the reflected EM waves. Every piece of matter has an associated electric permittivity and permeability. These can, and often do, change with temperature and the other environmental variables.