I've worked at Jane Street (opinions my own). Ocaml is only part of the retention tactic.

The implication that Jane Street locks engineers into only getting experience with Ocaml isn't really true: most engineers there are competent enough to keep another language up to scratch through hobby projects if they wanted to; and somewhere like the mentioned Millennium could still hire someone from Jane Street even with only modest C++ experience (for example) because of their domain understanding, low-latency experience, and general transferable programming skills.

It's just true that a lot of the people there genuinely value getting to use a functional language. Also they pay well, the culture is pretty good, and if something goes wrong you can usually just switch teams, so there's really not a big reason to leave.