I think tree-sitter's relationship with JavaScript is entirely syntactic. You don't need any JS runtime installed to write grammars, because technically tree-sitter CLI already has a JS runtime included and using that it converts your grammar first to an intermediate JSON format, then it generates parser code in C. And then this C code gets compiled into a shared library, which is what editors like Emacs use, so to use tree-sitter modules you definitely don't need a JS runtime either.