Scala without any optimization is still better than Java with all the ninja-skills black magic applied. Scala optimized is pretty good - "just" 2.5x slower than C++
Does that mean the java version would have been just as fast? It seems in general though that Java is just as fast as C++ unless you are an ultimate C++ expert.
If you read the paper, you'll note that the Scala author significantly changed the structure of the algorithm to conform with the way Scala does things (recursion, etc). So it's sort of an apples-to-oranges comparison as far as Scala's concerned, too bad they couldn't write ugly Scala code that would give a better comparison.