Sexprs are (usually) node-labeled trees (xml is too), while json describes edge-labeled trees. Object-oriented data structures are edge-labeled graphs. Json is a slightly better fit for the most common implementation languages.

The page you linked to doesn't support your argument, BTW. It tries to assert that sexprs support hashes - by extending the syntax with a reader macro!

This line shows the author's confusion: "S-expressions are more powerful (because of the duality of code and data)". The guy is confusing a format that strictly should not have behaviour beyond constructing a data structure, with Lisp more generally. If you allow the data structure to contain code that further interprets the data structure, the complexity of sanitizing input greatly increases.