Yes, that’s right, Diaz was there from beginning to end and did quite a bit of the killing himself. There were maybe 1 to 2,000 Spaniards in total at the peak but they were accompanied by tens of thousands of coastal tribe allies who were sick of Aztec arrogance and hegemony. Diaz notes on many occasions how bravely these allies fought. The number of Aztec warriors was great but not every single person in the empire was a warrior; they were an elite at the top of a subservient rural farmer population. So a population to Spanish soldier comparison (as seems to be alluded to in your British black propaganda reference) is invalid. The Quetzalcoatl myth may have helped get them into the island capital the first time but it certainly didn’t help them get out once the Aztecs turned on them nor did it help help them when they returned some months later, reinforced with more Spaniards and coastal tribe allies, to finish the job.
Disease had a role, as I acknowledged in my original posting. But the Spanish and their allies, together, took down the empire with the sword (steel and obsidian edged).
Yes, Diaz’ account is nearly the entire source for British black propaganda; British history takes Diaz’ evidently wild exaggerations and portrays them as typical of Spanish cruelty. Diaz’ book is nearly synonymous with “the Black Legend.”