He means something like "outside of the scope of Java".
Specifically in Java the "correct" way to create a new object is by using that Object's constructor. Many class writers rely on this assumption and code logic into their constructors - things like input validation or anything else you want to guarantee at construction time - this is what he calls "invariants established by the constructors". But cloning bypasses this basic constraint and creates a memory copy without invoking the constructor - hence it is "extra linguistic".
Technically, so does serialization.