Tim, your post includes two questions, without question marks, both of which imply a resounding yes, at least from your perspective.
1. “But isn't it the case that Heaney was always 'narrow'...”
2. “ but wasn’t it the case that the move to put some distance between the two poetries was made initially by the likes of Heaney, by which I do mean the likes of Heaney, and this happened quite a while before 1977...
In answer to the first I would say no, very definitely not. You’re as entitled to be unenthusiastic about Heaney’s poetry as David is to find it like viewing paintings from the Royal Academy, but you’ll know that many readers of Heaney, myself included, don’t share this view at all. The notion of ‘narrowness’ in the work of a poet isn’t incompatible with real quality or interest, but even then I find Heaney’s work far from narrow.
As regards the 2nd question, it’s very hard to make this claim convincingly, and it can’t be done on the basis of one story told about an event 40 years ago. And why do you want to lay the blame of the conflict on one side anyway, as in a kind of ‘he started it’ school playground brawl?