I spoke to John Blackley, a disciple of Murray's, who said the above comment is misinformation. He suspected the reason Murray withdrew from the rhythm debate was under obedience to his monastic superior. Blackley said Murray told him once, when he had asked Murray about it, that Murray had written all he needed to on the subject of chant rhythm.

But that's hardly saying he changed his mind.

Blackley's schola put out an album with mensural rhythm, whose insert contained a blurb by Murray where the latter said -- I paraphrase -- that he was grateful that efforts were now being made, as in the current album, to interpret the chant according to "contemporaneous treatises", i.e. the medieval Latin Theorists. That CD was in 1989, three years before Murray's death.

Furthermore, after a quick forum search, I've gained the impression from others' comments that this Paul Inwood has been a loud and opinionated person before, and that his opinions are not reliable. Until he produces the "complete set of writings" to prove his case, I think it's safe to say the comment above is untrustworthy.

If anyone has a list of Murray's "complete set of writings", please share it.