In Ocaml, interesting. I was similarly surprised when I learned that the firs Rust compiler was written in Ocaml, too https://users.rust-lang.org/t/understanding-how-the-rust-com...
SML is very dated and the standard library and ecosystem lack many things that are considered table stakes for a viable programming language nowadays. And F# and Scala are fine as enterprise languages, but being tied to .NET and Java respectively makes them less desirable for implementing a language that won't itself be coupled to one of those runtimes.
So OCaml was probably the most mainstream choice among the languages with appropriate tools, as funny as that sounds. And honestly, once you get over the syntax, it doesn’t actually have anything outrageous.