What happens at -80db isnt significant in terms of obtained measurement data.
The levels are not calibrated in that measurement so the absolute numbers are not correct. That aside you are right that for acoustic measurements, it is not consequential. For "THD" measurements however which is the topic we are discussing, the mic is adding its own distortion so you are not just measuring your speakers.
Retired Technology Insider
Founder, Madrona Digital "Insist on Quality Engineering"