But Femtolisp isn't Scheme, it's a non-standard Scheme with its own custom built-ins. It's quite impossible to run a Femtolisp program using a Scheme interprete

But Femtolisp isn't Scheme, it's a non-standard Scheme with its own custom built-ins. It's quite impossible to run a Femtolisp program using a Scheme interpreter without modification. Formal grammars are enormously useful, I almost always refer to them at some point when learning a language. It's an important part of being in an ecosystem larger than oneself. I'd question how a language without a well defined grammar has well defined semantics as the OP claims. How many languages have you learned the syntax of by reading the source code of its parser? How about when the parser is implemented on a language you've never used?