That statement is continually misquoted. The actual statement, from
Musicam Sacram, the implementation document is this:
- In sung liturgical services celebrated in Latin:
(a) Gregorian chant, as proper to the Roman liturgy, should be given pride of place, other things being equal.[34] Its melodies, contained in the “typical” editions, should be used, to the extent that this is possible.
(b) “It is also desirable that an edition be prepared containing simpler melodies, for use in smaller churches.”[36]
(c)
Other musical settings, written for one or more voices, be they taken from the traditional heritage or from new works, should be held in honor, encouraged and used as the occasion demands.[36]
- Pastors of souls, having taken into consideration pastoral usefulness and the character of their own language, should see whether parts of the heritage of sacred music, written in previous centuries for Latin texts, could also be conveniently used, not only in liturgical celebrations in Latin but also in those performed in the vernacular. There is nothing to prevent different parts in one and the same celebration being sung in different languages.
You are correct, for liturgies celebrated in Latin. But as noted, while taking into consideration the “pastoral usefulness” pastors should consider whether “…parts of the heritage of acred music…could also be conveniently used”, there is no mandate for them or for Gregorian Chant in particular in a vernacular Mass. Paragraph (c) also makes clear that using “new works” is also fully acceptable.
Another passage, which I’d have to dig out, also emphasizes that there is nothing wrong with using contemporary music.