According to Jörg Tiedemann, the author, it’s a paraphraser, rather than a summarizer. He writes,

There are texts with alternative translations into the same language, which I used for training intralingual models like this one. They are maybe not very useful at this moment as they probably just copy the input text. That this only obtains 40 BLEU is not that strange as this is tested with paraphrased sentences. Note also that this model is really rather a paraphrase model than a summarisation model as they seem to use it in the discussion