> If the Spanish hadn't ever set foot on shore and the Mesoamerican society was allowed to develop, they would likely have developed their own diseases and the subsequent immune response, which may have helped fight Smallpox.
The most charitable reading of this is "if cowpox, monkeypox or some other zoonotic pox virus had an opportunity to spread through Mesoamerican society, they might've fared better with smallpox."
A less charitable reading is "they might've faired better with smallpox if they had some other disease exposure prior to invasion."
Either of these cases seems implausible. I don't know of any pox virus from the Americas that provides immunity to smallpox. In Europe we know that cowpox provided immunity among milkmaids. We also know that it didn't spread between people. There's no reason to believe that the only missing element was time for some other pox to emerge. As for the less charitable reading, prior infection with a pathogen is unlikely to provide any benefit against an unrelated pathogen. In the decades after smallpox swept through Aztec society the Cocoliztli epidemics killed twice again as many people.
Fundamentally the problem was the introduction of a virus with a high mortality rate to an immunologically naive population. There's a major difference between a disease killing 50% of your under 12 population and 50% of everyone. The former is a disaster, the latter will collapse society, particularly one already fighting against an invasion.