I have no idea if this performance of the Machaut Mass is historically accurate or not. But it sounds absolutely 100x more plausible than any other performance I've ever heard.
Guillaume de Machaut is the first indisputably great composer of the classical tradition, the first true master of polyphony, and perhaps still the greatest of all French composers. He was born in 1300, the year in which Dante's Divine Comedy is set. Yet he is, in so many ways, more our contemporary than many later composers. He is a relic of the bygone era when composer and poet were one and the same. As a poet, Geoffrey Chaucer counted him as his primary inspiration. As a composer, his influence continues on figures as diverse as Steve Reich, Pierre Boulez and Brian Wilson.
Yet, as with so much Early Music, you'd never know how great a composer Machaut was from most performances. The fact that we get to hear music from 800 years ago is in itself a miracle. But so many performers of early music insist on a dry, academic performance style that is more reflective of research than imagination. There is no way to render an exact approximation (notice the contradiction in terms) of how music sounded 800 years ago, let alone 200. So an academic approach to music, devoid of risk, will always be reflective of the attitudes of our day rather than of the music's. As always, he only way to make this interesting is to inject your own personal interpretation.
I would not know if all these florid melismas are historically accurate. But they absolutely sound as though they could be. It sounds positively Arabic and places the Middle Ages French civilization from whence it sprung in direct relation to the Moorish civilization which sprung up to their south. It's a plausible, incisive, thrilling insight into history itself.