[octachron]

> This introduction doesn't really explain anything, as I guess it assumes you've learned OCaml elsewhere and are just here to practice.

Indeed the link is a not an introduction to OCaml but a demo instance of Learn-ocaml which is framework for building online exercise websites to complement (online or physical) lectures. This is why the instance is hosted on the OCaml Software Foundation website rather than ocaml.org: the demo is not targeted to learners but to teachers.