Most Slender Man stories tend to focus on the stalking and unknown aspect of the featureless monster.
This one does not, it may seem to attempt to explain too much about Slender Mane (and unlike normal Slender Mane stories, these are not guesses but accurate statements) and some people think it might detract from the horror.
That's not where the horror of this story comes from. The prime fear aspect is held up by the hopelessness of the situation. Twilight Sparkle is being haunted by an eldritch horror, and friendship cannot save her.
This helplessness is defined by when Slender Mane appears in the library over Twilight's bed and she screams and begs Spike to hit the lights, and when he does, he sees nothing, and patronizes her that it was only a nightmare.
Only two other people are even able to see the titular character, and one is far too afraid to help Twilight (and indeed, they abandoned Twilight to Slender Mane twice) and the other is not revealed until almost the end as a servant to Slender Mane.