In my undergrad years every major seemed to have what students referred to a "weed-out course." These courses were intentionally made difficult and often has inane non-standard grading policies (for example, in one, anything under 85 was considered an F). They were often taught on odd schedules (like one semester a year or something) and at unusual times. And they were often taught by difficult professors who did little, if anything, to help students succeed. If you were lucky, there might be a TA that might be able to help you, but students were often expected to form independent study groups and assist one another.

Basically if you couldn't pass this course - and in some cases department policy limited you to a single try - your chances of success in that major were slim to nonexistent, and you were encouraged by your advisor to find a different major. If you could pass the weed-out course, you could be reasonably assured you were ready to face the rest of your coursework, because nothing ahead would be as difficult as that course was. And, that you were reasonably certain this was what you wanted to do.

They weren't necessarily directly related to the major. For some of the engineering and science programs, a math course was often the weed-out course.

I can't say for sure that this was the author's experience, but her description of it sounds very much like a weed-out course.

I don't know if it's true of all schools, but I had a very similar experience at another Ivy League school. Took the first semester of linear algebra, got a very respectable grade, but really didn't learn much because the class wasn't structured to do that. It was there to make STEM majors prove they were good enough. I wasn't a STEM major, so didn't need to keep going with that kind of hazing.

Why not both? A particularly bad class demonstrating typical problems in an unusual intensity. Also an individual who had a particularly intense mismatch between her learning style and the course design.