Reread it. Jungmann clearly illsutrates it is a new rite that replaced other rites already existing- except those existing for 200 years
But the commission took another course, by establishing the wished-for uniform missal. This uniform missal was in truth a Roman Missal, for as its basis they chose the Missale secundum consuetudinem Romanae Curiae which already had the greatest vogue. ... For the exact rules about the choice and arrangement of each Mass formula and for the directions regarding the ritualistic aspect of the Mass, the Rubricae Generales Missalis and the Ritus servandus in celebratione Missae were prefaced to the new Mass book. These were taken almost bodily from the Ordo Missae of the papal master of ceremonies, John Burchard of Strassburg, a work which appeared in 1502 and had meanwhile circulated widely. (J. A. Jungmann, S.J., trans. F. A. Brunner, C. SS. R., The Mass of the Roman Rite, v. 1, p. 135)