This isn't true. I'm one of those people who tested remarkably well, and back in college would do fine on exams despite frantically copying all of my own (non-comp Sci) assignments. Better than my peers who knew more and helped me cram. Test anxiety is real.

I was a great test taker, I used to make a sort of game out finishing tests in half the time as almost everybody else and acing it at the same time. I also never crammed, never attended pre-test study groups, and sometimes made a show of drinking beers right before the test just to annoy the people cramming in the last minute.

But I'm not particularly brilliant, in fact I wouldn't be terribly surprised if I have undiagnosed ADHD. My test taking performance trick, which I freely told everybody to their annoyance, was very simple. I knew the material! Read the assigned texts, do the optional homework, pay attention in class. If you know the material you don't have to try to cram it into your brain in the last half hour before the test. If you know the material you don't have to try to reason it out from first principles during the test. You just go in, fill out the easy answers straight away, go back and do a second pass for the tricky questions, and that's it. If you have to sit there wracking your brain for 30 minutes on a single problem it's because you already fucked up with how you approached the course weeks ago.

Again, I'm not special for this. There were a handful of other students who were as fast as me. We'd sit in the hall waiting for our friends, look at each other and say "you knew all this stuff too, huh?" "yeah of course"

It is definitely not the case that if student A performs better on a timed high-stakes test than student B, that means A must have worked harder / prepared better / know the material better / etc. than B. Some people are very skilled at bullshitting their way through stupid school tests, and others are not. Very few school tests are well enough designed that they can effectively measure the intended target of how well someone understands the topic, content, and course-specific skills which are being intentionally trained in the course.

Bullshitting though tests is a learnable / trainable skill, but schools generally do not teach it very coherently or well and most students do not deliberately practice it. It generally doesn't have that much to do with the content or other skills intentionally taught by any particular course or by schools in general (there's decent overlap with the skills involved in competitive debate and extemporaneous speech, which some students participate in as an extracurricular activity). Rating students on how good they are at bullshitting their way through exams is sadly a significant part of the way our education system is focused and organized, but in my opinion it is not a valuable or particularly valid approach. There are certain professional contexts/tasks where this kind of skill is useful, but developing it per se shouldn't be the focus of the education system.

Sometimes this and related skills are summarized as "intelligence" ("oh she aced the test without studying, she must just be really smart", etc.), but in my opinion it's quite a misleading use of the word.