They are probably alluding to the tree-like structure of JSON where you can't represent cycles without inventing a pointer + identifier format first, and that i

They are probably alluding to the tree-like structure of JSON where you can't represent cycles without inventing a pointer + identifier format first, and that is indeed not part of the building blocks "JSON-the-language" itself provides. This seems to be a direct continuation of the SQL example -- wouldn't you be able to represent a tree with eg. tree paths stored for each item?