If all you found was an old API from half a decade ago that doesn't even exist anymore, than I think that's a pretty good case for Scala.

That's irrelevant. Scala simply allows your coworker to go "off the rails", employing their creativity even to language constructs, and you either spend time to learn their way of thinking, set firm rules about what is allowed, or you are simply out of luck, like often with C++.