What a strange statement/article. I had to check all three of those as an undergrad, associativity of matrix multiplication was definitely first year linear algebra, and I think elliptic curve product was first year number theory.

(and it still is, theorem 3.4 in these first year notes https://wwwf.imperial.ac.uk/~kb514/M1GLA.pdf)

"It is that they are felt too tedious or technical to prove in a course or even a textbook." Was my degree unusually tedious?