I'm not seeing much evidence of Dijkstra being unbelievably arrogant in those letters.

This mostly seems to be two stuck-up academics caught up in sniping (very cordially) at each other over petty academic crap.

EDIT: Okay, I've read EWD692 and I get why Backus was pissed now. This still feels a lot milder than people are suggesting.

Note that Backus's complaint is in relation to EWD638, where Dijkstra tears apart every claim made in a paper and accuses the authors of having a "pre-scientific attitude" and giving "the impression of anti-intellectualistic reactionaries". But none of this explains why the "provenance" of Alan Kay's statement about Dijkstra's arrogance is supplied in a retelling of Backus's statement about Dijkstra's arrogance. The provenance of Alan Kay's statement is that Alan Kay said it, in his OOPSLA keynote.