I'm not really sure what your point here. These are implementation details of how CL works. Has nothing at all to do with what s-expressions are in principle.
I'm not sure why it's so difficult to understand for you.
S-expressions give us a textual syntax for data. But not for Lisp. Lisp syntax is built on top of s-expressions and has to be learned. That's true for Scheme, too. See chapter 7.1 of the Scheme R7RS specification: "Formal syntax". That's not s-expressions.
> I'm not really sure what your point here.
That seems to be a common theme for you. I'll help you out: Your original points were this:
> Clojure syntax is s-expressions, your own private definition notwithstanding.
Which I showed you by pointing to Clojure.org to be wrong. Clojure syntax is more than s-expressions.
> The syntax is very minimal and consistent while also extremely expressive.
This also is wrong, since the syntax consists of all kinds special forms and macros, too. The user can even introduce lots of new syntax by implementing it as macros. Every library with macros is likely introducing new syntax and semantics.
Isn't it obvious? Reader macros are in the first place there to implement and extend the s-expression reader. It implements parsing of the data structures: conses, lists, arrays, vectors, strings, symbols, various number types, bit vectors, ...
