As someone who suffers from dysgraphia, which isn't well understood and commonly missed or completely misunderstood, our teaching methods simply do not translate to effective methods.
In English especially where we have problems with vowel shifts multiple source languages and frankly stupid spelling of some words by ignorant prescriptivist Latinophiles, it is really rote memorization.
Math is like playing the guitar or skiing, it takes practice to be good at.
But if you aren't good at memorization of concepts you don't understand, the typical response is to make people spend more time with memorization.
This simply doesn't work and students start to believe they aren't smart enough and give up, because they aren't offered tools to adapt and it is easier to just be thought of as stupid or slow.
Mathematics, logic, and scientific reasoning are learned skills and while everyone will have ultimate limits to their abilities, they are hard for all humans but in different ways.
But you are right, most this is tactic, people just ignore factors that often aren't even learning disabilities, but are advantages some students have due to their history and exposure.
Moravec's paradox is probably a good analogy, where what we think is easy is only easy due to previous experience.
Many authors like Ernest Hemingway and Agatha Christie were horrible at spelling and grammar but had good editors.
Assuming that being bad at spelling and grammar indicates an 'intellectual threshold' is a dangerous path.
Calculus is a filter class because it is taught in a way that favors rote memorization of unexplained concepts, but introduce some abstract algebra concepts to a student or coworker and many can move forward.
Rote memorization does have value, but equating memorization skills wilh ultimate intellectual ability leads to far lower educational attainment for many people.
Great guitarists and other difficult skills are far less about 'natural abilities' than it is about hard work and being exposed to an environment that does discourage that hard work.
Stem is the same as is creative writing or effective communication.
It is hard for everyone to learn, but people forget how much effort it took.