If it was a computer playing the exact same notes as MacDara Ó Raghallaigh but with simple sine waves, would it be boring then? The author notes in the first paragraph that if an instrument is producing overtones then it is being polytonal, and hence not necessarily boring.

Ack, I got so frustrated at the beginning of the paragraph I didn't notice the author was making up his own definitions of monotonic and polytonic, and instead assumed he meant monophonic and polyphonic. My bad.

Anyway, I suspect the answer to your question is no, if done properly. That is to say, I believe if the computer caught all the rhythmic, dynamic, and pitch subtleties of the music, the music would still be interesting, if perhaps harder on the ears.

A fiddle is not monotonic.

Indeed. From the end of the third paragraph:

  And don't think that you need more than one instrument
  or more than one voice to make real music. Pretty much
  every musical instrument and every person's voice is a
  source of polytonic sound.