That's a bit overstatement. You can call .Net code from F#, just like you can call Java from Clojure or Python/Rust/C from OCaml.
It just would be foreign and unidiomatic (hello, null pointers, methods, OOP).
And you can call C/Python/Rust from OCaml, we made quite a lot of wrappers for Gstreamer and some Rust libs in OCaml and it was quite painless.
So it's a bit of simplification, you can call foreign code in OCaml, it's not that much harder than F# considering there are higher level FFIs for C, Rust and Python available. Author could just use these as well.