Earlier quoted context omitted.
Poking around in the sources, it looks like this is the OCaml compiler, with the frontend apparently tweaked to accept the new syntax. But this is not mentioned anywhere that I can see. The "Copyright copyright 2017-2018 Philip Blair and Oscar Spencer." line in the README is highly misleading in this context, since most of the actual source files are marked with OCaml's copyright header.
Many files are taken from the OCaml compiler and then adapted, but changes seems a bit deeper than just a different syntax. It would indeed seem fair for the authors to at least make it clear in the toplevel README that the front-end (parsetree representation and type-checking) is indeed / started as a fork of the OCaml code base. Considering ongoing efforts to create a WebAssembly backend for OCaml (and thus Reason)…
Aside from the wasm compilation, I don't see why this exists.