I see! "Built on X" would usually refer to what language something is built in, while what you're talking about is "interoperability" with the host language.
But yeah, unfortunately Joker doesn't do interop, which is sad because it's one of the coolest features of the Clojure language.
Joker is a Clojure interpreter, mostly used as a linter in the community but can be used as a runtime as well.