I checked the Boethius cite, years ago, and I remember agreeing that the case for the dependence of one text on the other was strong. If I recall properly, it involved the words "suaviter disponens omnia". But in any case I know of no way to decide, without further data, whether Boethius was copying an existing antiphon. Perhaps whoever wrote the antiphon text cribbed the words from Boethius.

T.