Sure, the body absorbs waves of this frequency, but all it amounts to is those waves being able to vibrate some polar/water molecules at most which would generate heat. Just standing outside is probably more harmful because you’d be actually taking in UV radiation which is actually ionizing.
Stuff like Wifi that is in the 2.4 GHz frequency range, has a wavelength of 125 million nanometers which corresponds to 1/100,000 of an electron volt, whereas ultra violet is anywhere between 3.1-10 eV.
A human body emits infrared at a wavelength of 12000 nanometers, which corresponds to 0.1eV. So just standing next to someone would result in more energy transfer/absorbtion than your regular wifi.
If I accept that is has harmful effects, then how harmful are these effects relative to other things? Just saying you have a higher chance of contracting or developing something doesn’t mean anything if the chance is already low. It’s very easy to make statistics and numbers misleading. The excerpt you quoted for example mentions they use a “sub-THz” band for their experiment. THz is 10^12, whereas MHz is 10^6, THz isn’t even used as a business band, and the highest frequency is like 500MHz.