Femtolisp is a very small, fast Scheme implementation – and Scheme is the most thoroughly standardized dialect of Lisp there is. The fact that Julia's parser is implemented in flisp also completely irrelevant to the user – you can for all purposes just consider it a C program that implements a custom parser for Julia. It's true that there isn't a formal grammar specification of Julia's grammar, but I'm not sure what that would be especially useful for.
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?
Learning a programming language by looking at the formal grammar is like learning a spoken language just by reading a grammar textbook. No one actually does that. Learning a language by reading the parser is even more ridiculous. That's like learning a spoken language by dissecting the brain of a native speaker. The way people actually learn languages – both programming and natural – is by seeing examples, reading explanations, and by trying things out.
