The code in the screenshot/asciinema isn't s-expr by any common conception of the term, or as a Lisp would parse it.

  (flatten '( '(a '( b)) '( c   d)))

  '((quote (a (quote (b)))) (quote c d))

  '(quote a quote b quote c d)

Parsing sexprs is quite easy, no need to use a complex parser generator. The core of parsing sexprs is you have a "read_list" function, which accumulates values until it finds a ')', and whenever you encounter a '(' character, you recurse into that same read_list function. You don't even need a lexer.

Also, this is divided in 8 crates for some reason... don't take this the wrong way, but is it the result of vibe coding?

Doesn’t seem vibe coded to me. The crates aren’t unreasonable: data structures, parser, formatter, repl, “util,” some proc macros (which have to be in a separate crate in rust), and a VM.