While it is has been recognized for years that human hearing is not very sensitive to low bass frequencies, which must be reproduced with much more power and intensity in order to be heard, what these results show is that
our detection threshold for noise (made up of harmonically related and non-harmonically related test tones) is practically non-existent at low frequencies. (The noise test tones are noise in the sense that they are not musically related to tones commonly found in musical instruments.)
In fact, the noise tones at 20 Hz and 40 Hz had to be increased to levels louder than the music itself before we even noticed them. Put another way, our ability to hear the test frequency noise tones at frequencies of 40 Hz and below is extremely crude. Indeed, the results show
we are virtually deaf to these distortions at those frequencies. Even in the mid-bass at 280 Hz and lower, the noise can be around -14 dB (20% distortion), about half as loud as the music itself, before we hear it.
https://www.axiomaudio.com/blog/distortion