Well, the most recent Apostolic Visit of Benedict XVI provided an excellent illustration that this is not the case. Here is the communion proper Dilexisti sung in Domplatz by a Gregorian choir.
Incidentally, they are single from a single edition of chant that is really a blown-up version from the Graduale Triplex, with the antiphon on the left and hand-written Psalms on the right.
The chant itself is certainly, then, first millennium.
"Thou lovest justice and hatest wickedness. Therefore God, thy God, hath anointed thee."
Here is a clean edition.
![](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhquc5h6H8-Zh66iuGZ_78bPii623UNK2KB0I4hA98HV1Js4QOWu6w3jK-zJhTSTrxpG0eLgmszeI8FbwB8seg3beCHEts41l9LbZ6S9j_t6En0-p6CKd-f5xulUi54lY8raogX/s400-rw/dilexisti.png)