To begin with, St. Augustine doesn’t say this. The quotation is quite certainly from a (presumably medieval) spurious work attributed to Augustine, known as “Sermo de adsumptione sanctae Mariae” and counted as “Sermon 208” in that collection. 1/2

Pope Shenouda explains the use of this symbolic surname of the Virgin in the same section: it is the application of Psa. 45 - as prophecy - to her (similar to the Prophetic presentation of YHWH as Groom and Israel as Bride). The same is the context for Anba Raphael's sermon. 2/2

Moreover, this exegetical understanding of Psa. 45 is a natural extension - if not an existent aspect - of the Lukan narrative: I will probably write a thread on this matter soon.