I don't think that's what the article is talking about: differences between perfect minor 7ths and equal-temperament ones. Rather, the tuner in question basically has already nailed the equal-temperament minor sevenths already (as far as you or I might think) and is just listening for beats among their harmonics now, so that when a run of parallel minor sevenths is played, the frequency of the beats among the harmonics rises in proportion to the frequency of the chord, like speeding up a recording.