The key word in the case is harrasment. Nobody would have written about this case if he had mailed hundreds of menacing letters using the postal service.......
Funny thing about social media - lots of people seems to forget at that they are responsible for their writings even though it may appear that they are writing annonymous. They can be found and writing hundreds of menacing tweets to someone is harrasment just as if they had written them all down on paper and mailed it.
He will probably only get a fine but at least he will learn that even tweets are subject to harrasment and slander laws.