I am not sure I believe that :) Singe tones should show up noise artifacts better. IMO, -66dB should be just fine for the alleged application.
Let's presume you're using it in a busy pubic space. Room tone plus the hum of activity should be more than50-60dB. If you make the beeps 90dB, then the noise is -20 relative to room tone.
That is not enough to be completely masked, but it's close.