"Second-harmonic distortion is exactly the same note, an octave above. Ditto for higher-order even harmonics; they are also the same note more octaves above. "
From the sentences immediately prior to that:
"
Tube amplifiers have much more distortion than solid-state amplifiers, but most of it is second-order, which is quite musical. That's why it's called "harmonic" distortion. "
Well, the first phrase in red is often (not always true). the rest of that is incorrect.
From the sentences immediately after that:
"Even-order harmonic distortion can be so pleasant that back in the 1970s the Aphex Aural Exciter was very popular in recording and broadcast specifically because it was designed to generate and add these harmonic distortions! "
Also incorrect, that is not how the Aphex worked.
You really have to dig to find anything correct in this piece.