Harvard’s lowest level math courses are:
Math Ma, Mb: two semester single variable calculus sequence equivalent to 1a with review of precalculus
Math 1a, 1b: ordinary two semester single variable calculus sequence
In other words, these are suitable for those with ordinary readiness for college level math (Ma and Mb are for those who are a bit behind).
It is the subsequent courses, for those who have had single variable calculus while in high school, where there are five levels:
Math 21a, 21b: ordinary multivariable calculus, linear algebra, differential equations – appears similar to ordinary sophomore level math elsewhere
Math 22a, 22b: 21a, 21b with more rigor
Math 23a, 23b: linear algebra and real analysis
Math 25a, 25b: theoretical linear algebra and real analysis
Math 55a, 55b: studies in algebra, group theory, real and complex analysis
The latter two sequences require familiarity with proofs (i.e. beyond just 5 on AP calculus BC or equivalent that the first three sequences list).
http://www.math.harvard.edu/courses/2018-2019.pdf
In other words, Harvard offers “ordinary” math courses (1a, 1b, 21a, 21b) as well as a semi-remedial course (Ma, Mb), in addition to four levels of honors math courses. It does not offer low-remedial math courses (e.g. elementary algebra, high school level geometry, intermediate algebra), but it is presumed that someone who can get admitted to Harvard (even with the greatest hooks) is beyond that. But the minimum math skill in the lower level math courses at Harvard is ordinary readiness for college level math, not top-end math skill like at Caltech and Harvey Mudd.