This passage is of vital importance because it leaves no doubt that King Arthur's Avalon was NOT given as a synonym Glastonbury. All allusions linking Glastonbury to the Isle of Avalon during King Arthur's historic era came after Malmsbury, whose floruit was 1110 to 1140.
According to John Scott, the interpolations of the DA are distinguishable from Monmouth's original because of 1) the use of different pens, 2) the use of simplistic grammatical structure which contrasted to Malmsbury's more complex use, and 3) the poor Latinity. Scott is confident about interpolations when he writes, "It can be discerned easily that at least two different monks were involved."
*See Chapter 8, "The Isle of Avalon," pages 221-262 in The Historic King Arthur and pages 5, 53, 200 in Historic Figures of the Arthurian Era.