I'd say this depends of the complexity of the I/O system.
This was the original claim, which you supported:
'Parsing Lisp in Lisp is so easy because it’s free.'
The example you were pointing to is explicitly calling a parsing engine of Racket via 'read-syntax'. Actually more complicated than the usual s-expression reader - which does only read s-expressions, but has no further idea about Scheme syntax.
Check the usual Scheme report / Racket documentation for the definition of Scheme syntax, syntax objects and its extension mechanisms (macros, ...). I'd say the whole thing is non-trivial. There is a grammar of Scheme, but it is not fixed, because there are extension mechanisms, which make parsing challenging.
It's 'free' because it's a provided language facility - but not free in terms of complexity of the concepts to understand.
And no, the syntax of s-expressions (-> data) is not the syntax of Lisp. It's just the syntax of s-expressions. Search the Scheme report for 'syntax'...