Complex sentence: To build chords, Fripp uses "perfect intervals in fourths, fifths and octaves", so avoiding minors and major thirds, which are implemented poorly in equal temperament tuning (in comparison to thirds in just intonation).
Split into simpler sentences: To build chords, Fripp uses "perfect intervals in fourths, fifths and octaves", so avoiding minor and major thirds. Major thirds and minor thirds are implemented poorly in equal temperament tuning (in comparison to thirds in just intonation).