Anonymous IV (who is usually believed to have been from England) mentions that thirds were considered discordant. I can't help thinking that this is rather like all those Papal Bulls forbidding dancing in church... if it wasn't happening in practice, there wouldn't have been the need for a ban on it
He certainly
may have been English (though some dispute this), but he was certainly educated in Paris (which is why he is the main source of theoretical information regarding the work of Perotinus and Leoninus). I don't think there were many thirds and sixths flowing though the cathedral of Notre Dame at that time (except as 'passing dissonances'), and he may here be showing his Gallic influences.
Baz
