Saying that that makes it ineligible for being a copyrighted work on it's own is like saying that Microsoft owns Flash because the runtime is ported to IE.
Look, I have never heard of an add-on for Blockland that is not made to compliment Blockland. That is the definition of an add-on.
If you want to scrutinize minuscule potential loopholes in what is a de facto rule, go ahead. But if you brought up some add-on to a Judge that had no idea what the forget an "API" is and tried to pass your Blockland add-on as Intellectual Property, she'd laugh in your face.