The point that Guns makes is the overall point that the Indo-Euro society was itself the recipient of the fortune of an east-west vs north-south agricultural belt and the prevalence of easily domesticated food and farm animals which also led to "superior" diseases like smallpox. His thesis is that is was basically their luck geographically speaking that led to their eventual global dominance and not because of some racial characteristic of that God was on their side or whatnot. Then mameluksabre and I got a bit sidetracked by the particular question of Cortez's "luck." But, truly part of his luck was that he was from a society with superior military technology... based again on Europe's general luck with how their land happened to be formed.
I still maintain that Cortez was a slick guy and that it wasn't his horse nor his guns that were nearly as effective as his lies and manipulations. He was truly one of the greatest leaders of all time, and it was his leadership that made the impossible inevitable. Not to get all Ayn Rand, but Cortez is certainly a great example of how only the individual can truly change the world. The aztecs were more advanced as a society(notwithstanding the guns), but they lacked this fire of the individual... and so they perished as socialism always will in the face of a great man.