alt Hacker NewsI think you misunderstood the comment you're replying to.
They meant (like Wikipedia says) that traditionally a tetrachord spans a perfect fourth (i.e., that's the interval between the first and last notes), not that it's a stack of perfect fourths.
As to the original point, any discussion about dissonance of chords is meaningless without mention of temperament: in the modern tuning system (equal temperament[1]) both major and minor triads are pretty dissonant, look at the thirds at the table in the section named "Comparison with just intonation" in that Wikipedia page.
We have been dealing with the tension between keeping thirds and fifths pleasant for centuries: very roughly, people used meantone temperaments[2] in the Renaissance (which had perfectly tuned thirds), then came the era of well temperament[3], and eventually we settled for the simpler equal temperament we use today with its nasty thirds (it has several other advantages, but "having pleasant major and minor triads" is not one of them).
[1] https://en.wikipedia.org/wiki/Equal_temperament