I'm a farmer, not a teacher, so I can't answer this question as asked.
But I would speculate that of the set of people who bombed the class, they could fall into a number of buckets. E.g., one bucket is people who were mentally capable of learning the concepts, but were to lazy to put in the effort (then we can quibble about whether inherent laziness puts people into the "not capable" bucket). Another bucket is people for whom alternative learning environments might have brought them to understanding and a passing grade. Another bucket is people who just lacked the preliminary background and with a couple years of effort could be made to pass the class as it exists. And finally, another bucket is people who are genuinely incapable, regardless of environment, of understanding the concepts.
This shouldn't be surprising. I have tried to deeply understand quantum mechanics, and while I can parrot some of the most well-known and more simple concepts, I truly believe that I lack the capability of grasping the very core, deep insights in an intuitive way. I might pass undergraduate level classes in the topic, but I am fairly certain I couldn't achieve a PhD. I'm not the dumbest bulb in the shed, but I can see that there are people much, much brighter than I, and it is obvious that their ability to understand more advanced and deep concepts is greater than mine; This leads to the observation that of the set of understandable knowledge in the universe, some of it is available to some people but not available to me, no matter how hard I try. (I take solace in Feynman's quote, "If you think you understand quantum mechanics, you don't understand quantum mechanics.")
Then look at every human capability and its distribution across the universe of humans, and it's pretty clear that we all can't do everything, every one person has some cutoff beyond which they aren't capable of understanding in any given topic. For some people (and hey, maybe it's a really small slice of the population), that cutoff is somewhere before Calc II.
I think we're getting closer to the actual situation with this description. There are a lot of buckets of people who don't do well in a particular class.
And we both admittedly don't know the size of the bucket of people who cannot, given a lifetime of 80 years continuous study and tutoring, understand a topic.
But in my teaching experience, it's dwarfed by the group of people who doesn't care about the topic and bombs because they don't put in the effort.
