Okay. I wouldn't call that a persistent tradition but it's something better -- a modern theory that can stand on its own terms.
We already know the theory mac Duib evolved to MacDuff isn't original to Bannerman. The idea the the MacDuffs took their name from King Dubh goes back at least to the late 1800s, if not further.
And, it's certainly true that many Scottish clans were "founded by" a grandson (or great grandson) of their eponym. That is a point I made earlier about the problem with Ethelred being the name father of the later MacDuffs. Maven, I think you noticed it as well when you pointed out that Duff mac Eth has to be a fictitious person constructed to give the later MacDuffs a new eponym.
My response to Bannerman on this point would be that we don't actually know who the first MacDuff was, so guessing that the name father was grandson Giric then arguing that Giric was the founder is a circular argument.
The ancestor could have been any of King Dubh's grandsons or great grandsons. And, following the pattern of other clans, If Giric was the eponymous ancestor, it is likely his collateral kin also descended from Dubh were included in the label MacDuff.
In other words, Bannerman's argument is sound but it only goes so far. Giric is a candidate, but not the only candidate.