I suspect this "not a systems language" alludes only to OCaml's rather steeper learning curve and until-recently difficulty with multiple threads. I am sure it could roll just fine as a single-threaded compiler language written by a small team, which indeed, it was.

I wrote a toy Scheme implementation in OCaml by using the Camplp4 preprocessor. In benchmarks, it was faster than Gambit Scheme, which compiles through C.