Jane Street first introuced OCaml as a temporary prototype language that happened to stick around. They consider it a good decision, but one that happened by accident!
They’ve since grown (from 30 -> 800 people), and make language/compiler-level contributions themselves.
A framework to think about how language choice affects an organization:
Performance
Safety
Productivity
Learnability
Many additions to OCaml have been useful to Jane Street; some they’ve asked for/implemented, but others they didn’t realize they needed.
Some of the latter (not sure I understand most of these):
First-class modules allow you to work with modules as regular values at runtime, performing module-level dynamic dispatch (as an example) at runtime.
Merlin provides IDE-like features in regular text editors; the speaker was completely ok using OCaml without this sort of tooling until it existed, at which point the old way seemed archaic. I’ve definitely seen this in practice a bunch.
PPXs improve OCaml’s metaprogramming capabilities (admittedly not as good as Lisp{-like} languages).