This is about a decade old, I think. With a substantial optimisation suite added to ocamlopt since, some of it will be out of date. The advice on inlining in particular is obsolete.
[0]: https://github.com/janestreet/janestreet.github.com/commit/7...
If you're using OPAM, install an flambda-enabled compiler with (e.g.)
opam switch install 4.04.1+flambda
[1] https://caml.inria.fr/pub/docs/manual-ocaml/flambda.html