I can't speak about UCSD, but from talking with Harvard Math MA students and seeing their homework, it's actually some of the best designed pedagogical math classes I have ever seen. And it's definitely more rigorous than the average middle school or high school math class for sure.

These broad claims about the nature of these "remedial" math classes in the article probably need more detailed evidence, because for example the acclaimed IMO problems (that AI companies tout their models with) are also algebra and pre-calculus problems and can be quite challenging and intellectually engaging. Developing a good math foundation without calculus for a humanities or political science major who never had a good math class in high school is a noble goal, and probably very beneficial for them and society.