In 1950, the scientific community had already recognized that evolutionary theory implied polygenism. This wasn't unknown to Pius; in fact, the section is intended as a response to the scientific consensus, which on this point was largely the same as it is today.Development of doctrinal understanding, not rejection.
If a 12th century pope, let's say, believed on the basis of Aristotelian science that the motion of heavens were moved by unmoved movers in the form of spiritual aether beings and derived theological proscriptions or prescriptions from that flawed understanding, are his thoughts binding? No, they aren't - because the principles are deduced from premises subsequently found to be wrong, and those premises are not within the remit of divine revelation.
The changes on this point haven't been a matter of science so much as matters of politics and culture. Anti-science views are now an embarrassing liability.
I don't accept the idea that Pius was incapable of expressing what he meant. I also don't accept the revisionist idea that changes in Catholic thought since Pius wrote the encyclical had an influence on what Pius intended to express.If the premise of his understanding had been as developed as today and he left no doubt about that, I would have supported the conclusions he derived.
But as the premise has been overtaken by subsequent advancements in science, I do reject the prescriptions flowing from it. Yes.
The issue is now unsettled by the church on account of the advancements in scientific knowledge and theology that I've just mentioned.
Your treating Humani Generis like a fundamentalist Protestant treats the Bible, like an eternal fossil incapable of any progression, whereas it was actually a historically contingent document referring to a set of issues based upon the fragmentary knowledge of the era.
It's perfectly reasonable to say that he was sincere but mistaken and set his opinion aside.