The systema teleion is evidence of their compository technique and probably philosophy, but the inclusion of the diazeuxis shows that even they realized there were issues with harmony if their system was used as-is. In any case, what we'd need is not just evidence that they used the systema teleion generally, but that it was used for harmony, that the harmonies between octaves and other notes was considered equivalent, and that this usage was accepted ecclesially. Is there evidence of any of those 3 links in the logical chain?
I am not arguing that the suitability of polyphony for worship can be demonstrated by the music of the Ancient Greeks. The original point of contention here was that you have assumed octave equivalence is a universal feature of music (so that you can except men and women singing together at the octave from being polyphony sensu lato), and I do not accept this premise. The systema teleion was being presented as evidence against the universality octave equivalence, for the Ancient Greeks did not have repeating note names at the octave.
Thanks for the info about the date of the solmization. It is not surprising that the change to chromatic is a late one, meaning that clear octaves were present in the earlier music.
That does not at all follow. The first example I gave of non-equivalence at the octave was not in the chromatic genus but in the diatonic genus, which has been present throughout the entire history of ecclesiastical music. If one takes just intonation as the basis for the tuning of the diatonic scale (as is described in some theoretical treatises) the intervals are the greater tone (μείζων τόνος), which is identical in size with the Pythagorean τόνος, being a ratio of 8:9, the lesser tone (ἐλάσσων τόνος), being defined as a ratio of 9:10, and the least tone (ἐλάχιστος τόνος) which is defined as a ratio of 15:16. The the interval between ananes and neanes is a lesser tone, between neanes and nana a least tone, between nana and agia a greater tone, and between agia and ananes a greater tone. Thus when we go from neanes up seven notes, we ascend least, greater, greater, lesser, least, greater, greater, ending on ananes. Expressed mathematically, that is (15/16)*(8/9)*(8/9)*(9/10)*(15/16)*(8/9)*(8/9) = 40/81, which is wider than the octave by one syntonic comma (a ratio of 80:81). From nana, we ascend greater, greater, lesser, least, greater, greater, lesser, ending on neanes, which yields a ratio of 64:135, which exceeds the octave by a greater tone less a least tone, an interval which is about the size of an equal tempered semitone (being just 8 cents narrower).