Something Beautiful to Start off Advent

This is the "Missa Orbis Factor," a Latin setting for the Mass whose title means 'Maker of the World' Mass. I found it at the Catholic Key blog. Every mention of it I find on the web says that it is Gregorian chant. All of these sources are amateurish, though - bloggers like me. It strikes me as simpler than Gregorian, though. It sounds more like Sarum to me. This theory strikes me as likely because it seems to be an old English chant, or at least is making its appearance through Anglican parishes. Except, well, that could just be because they like beautiful things (may they bring them with them when they come)! But then, I am amateurish - just a blogger. The biggest piece of counter-evidence to my theory is that the chanting in the YouTube video I have enclosed is actually performed by a Milanese Gregorian schola. One would think they would know... except maybe they like to branch out. Plus, it's not like Gregorian and Sarum are worlds apart... not to my amateurish ears, anyway. Anyone out there know more?