They've spoken about this before and might not have picked OCaml if they had to make the decision again today. I think at the time it made the most sense.They can also make it work in any language as they claim to only recruit the best and brightest in the world (this may be the truth). My opinion is they could have chosen many languages and had similar results while balancing different trade-offs.

Regarding attracting talent, they've said they don't care about existing knowledge of OCaml as the language part is something they train staff on anyway. Their interviews are brutal from what I recall. I could be an OCaml expert and have no chance of making it through an interview without equal talent in high performance fintech and relevant mathematics.