Could be, but this implies that any distortion below -50 or so is inaudible, making me question the validity of the claim. If I play 1 khz at 0 dB and 3 khz at -50, is the latter inaudible?
It depends on how far the harmonics are removed from the fundamental and at which frequency it is (masking).
Tones and music are different things.
A S/N ratio of 50dB is audible because the average level of music will be 10-20dB below peak from which S/N ration is calculated.
Noise will thus be 35dB down below average music levels.
It also will depend on how loud the peak/average SPL is.
In music our dynamic range is about 70dB, add 20dB and 90dB S/N ration is fine.
Unless you have gear that can reach 120dB peak and you want absolute silence standing close to the speakers. In that case you may need 120dB S/N ratio because standing close to the speakers you want silence but you listen further away (lowering the noise at the listening position)
Also those that listen for noise 10cm away from their tweeter but listen 2 meters away to music will need a much higher S/N ratio than those that only care about noise at the listening position. The picture of the working men illustrates this well.
When your distortion products are not masked they also need to be low but because these distortion products are narrow in BW and noise is wide distortion can be a bit higher than noise before it becomes audible. For nasty sound you need substantial amounts of distortion (depending on music content as well because this can mask)
Some folks are bothered by noise, others less so. Also noise spectrum matters. It's complex, certainly with music.
Needless to say... It is not extremely costly nor difficult to build electronics with low noise and distortion (high SINAD). For vinyl and mics this is another matter and getting low noise is a bit more demanding/difficult.