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), I wonder what would be the selling point of Grain.