Maybe I’m obsessing. Probably. But in any case, I felt validated when these two volumes arrived at my door from Italy.
Here’s what he says:
And here is his evidence:
Francis Gignac, A Grammar of the Greek Papayri of the Roman and Byzantine Periods (Vol. 1; Milano: Istituo Editoriale Cisalpino, 1976), 86, 88-89.
All in all, in light of this, I’d say our English-ization of “dipthong” (δίφθογγος) hasn’t missed the boat on its pronunciation here.