> Lisp macros are meant to rewrite S-expressions that represent source code

'source code' is slightly misleading, since the s-expressions don't need to be valid source code. What's valid is only limited by what the macro wants do. If a macro accepts the s-expression form, then it is valid.

For example that means that the macro form itself does not need to be parseable to an AST by an existing language grammar. The macro itself can parse the s-expression in arbitrary ways. If data is source for the macro, then it may do something with it...