Why is that crazy? I've never thought myself, or heard anyone imply, that the "disease holocaust" was intentional. As neither nation had the knowledge/science to understand diseases/germs at that time it seems straightforward to assume it was accidental
And it's certainly true they killed a lot of the survivors, but most people just died of "natural" causes. Many before the Spanish even got to their areas.
Had the immunological playing field been even, there is no way the Europeans conquer the Americas in the 1500s. It took until the late 1800s until they could overpower the less developed Africa.
So think about what that world history could have looked like!!
When sieging cities in antiquity water supplies would be contaminated. Arrow heads be smeared with poison and otherwise feces to cause infections. In Europe during Middle Ages corpses and feces of diseased would be catipulted into cities to cause infection, notably this this even happened with corpses that died of the bubonic plague.