Agreed. Human thinking is arbitrarily high-order -- we use statements about statements about statements with no particular natural complexity limit. This seems to me the big limitation of knowledge graphs: The majority of real-world information, just like the majority of natural-language sentences, are highly nested relationships among relationships.
That was my motivation for writing Hode[1], the Higher-Order Data Editor. It lets you represent arbitrarily nested relationships, of any arity (number of members). It lets you cursor around data to view neighboring data, and it offers a query language that is, I believe, as close as possible to ordinary natural language.
(Hode has no inference engine, and I don't call it an AI project -- but it seems relevant enough to warrant a plug.)