Pianos should not have that - a piano should have a minor 7th that doesn't beat at all. However if you play the key tuned as a perfect minor 7th and use it as the root note for a standard major chord (root-third-fifth) it will sound terrible as the intervals will not even be close. Your choice is have hundreds of keys on the piano so that when you want the chord you select the exact perfect pitch you want (called just intonation), or we have to compromise the tuning of the piano somehow so we can get by with just 88 keys. We have chosen to stick with 88 keys and then use equal-temperment as a compromise so that the key we use for a minor 7th also sounds okay when used as the root note in a regular major chord. The piano overall sounds much worse than perfect, but it is an acceptable compromise. there are other compromises that people sometimes argue are better, whole books have been written on the subject.

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.