This is the great question and no one knows the answer for sure. I think we can say that it probably did not sound the way we sing it now, with the idealization trends that came into focus in the late 19th century. To me, to discover that the way we sing now chant is not the same as it was sung in the 7th century is not a case against the way we do it at all. Idealization and development is good, and adaption to time and place is as unobjectionable as it is inevitable. Music must also be heard and understood by others; it’s not only about the performers and their theories. In any case, I’m happy with a world in which a thousand chant styles and approaches bloom.
All that said, I wonder whether your intuition accords with the intuition of these performers. Did chant for the Roman Rite once sound this way?