They didn't give the book just the alledged source. They love dialogue but need substance once they are challenged - which is fair. Looks like one of my guesses is leading the field.
McGrath of Harlow - that Roman of yours said voce maximus est dexter - Latin scholars discuss.
Though we should remember that Roman military was constructed of conscript/mercenary cohorts and most spoke Greeko/Tuetonic and I wouldn't begin to guess on those.