Oh, you're right, it's unintended. I didn't see the author tried to trigger a type-check error with an argument "a" of the wrong type.
However, failing compilation because you're calling a non-static method from a static context is also a kind of static checking, so the author's example still works, just in unintented ways.
Even if the type checker passed the compilation would fail because it's trying to call a non static method from a static method