You're confusing ignorance with stupidity. People at the time were coming to the best conclusions they could based on the evidence they had. That isn't stupid. If humans were truly "incredibly, unimaginably stupid" we wouldn't have even gotten to the point of creating agriculture, much less splitting the atom. We didn't get here through "miracles," we got here through hard work and intelligence.
Stupid is people in 2025 believing the world is flat and germ theory is a hoax. Ignorance becomes stupidity when our species stands on the shoulders of giants but some people simply refuse to open their eyes.
Ignorance is when you don’t know something. Stupidity is when you think you know something and are presented with evidence to the contrary, but you dismiss it because of something stupid (i.e., irrational).
Of course, all these words have some overlap. My larger point is, people rarely come to rational conclusions organically, and it takes decades to centuries for even the most empirically verifiable idea to permeate, especially in the face of misinformation campaigns or when against “common sense”.