This was mooted back in 2011 or thereabouts as some others preceded you in your interest on this point, and the relevant staff at the USCCB indicated the use of the term intended no such restrictive meaning. And that was that.
Since the first source for the propers is the Graduale Romanum, it's only natural that the term for singing the Mass is "chant." The Latin word isn't "canticum," which would readily correspond to "canticle" or "song."
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