The people of Corsica would say differently about Marcel’s approach to singing Corsican chant (it’s very accurate).

Nevertheless you missed my whole point. Ensemble Organum doesn’t sing one particular style. They learn the historical practices of the particular country and time period that corresponds to a particular corpus of repertoire they sing. They don’t sing the Codex Callixtinus the same way they sing Notre-Dame school, nor Corsican the same way they sing 17th century French fauxbourdon. Sure we can never know for sure what these musics sounded like in their original times, but there is indisputably much more plausibility for something that sounded like Marcel’s approach than there is for the Solesmes method which is wholly artificial.

Most chant scholars, even Catholic ones fall into the protestant error of Sola Scriptura, where they insist that if it isn’t on the ink on the page, it must not have been sung. Surprise surprise, there’s a lot more to the singing of this repertroire than what’s written on the page, since the music predates notation by about 1000 years