An "AST in spirit" is in our mind, but not the machine.

    (let ((s (s s)))
     (flet ((s (s) (declare (type s s)) s))
      (tagbody (s) s (go s))))

The function READ has no idea what S is: variable, operator name, data symbol, type, go tag, function name, macro name, ...? For each above we don't know what s is. We would need to parse it according to some syntax (and possibly know the current state of the runtime.

An AST would be the product of a parser and the parse would parse the code above according to some provided syntax. The AST then would encode a tree built from some syntax, where the nodes would be classified. In Lisp symbols and lists have multiple uses and the s-expression does not encode which uses it is: is it data, is it some kind of syntactical element. There is also no defined parser and no syntax it encodes on that level. READ is at best an s-expression reader, where it has zero knowledge what the lists and symbols supposed to mean in Lisp. For the reader (+ a b), (a + b), (a b +) are just s-expressions, not Lisp code.