One of the most important and beneficial trainings I ever underwent as a young writer was trying to script a comic. I had to cut down all of my dialogue to fit into speech bubbles. I was staring closely at each sentence and striking out any word I could.
If you think you can point to an unnecessary sentence within it, go ahead and try. Having a long story isn't the same fundamental kind of issue as having an extra sentence.
How about the second sentence1?
Every inch of wall space is covered by a bookcase. Each bookcase has six shelves, going almost to the ceiling. Some bookshelves are stacked to the brim with hardback books: science, maths, history, and everything else. Other shelves have two layers of paperback science fiction, with the back layer of books propped up on old tissue boxes or lengths of wood, so that you can see the back layer of books above the books in front. And it still isn't enough. Books are overflowing onto the tables and the sofas and making little heaps under the windows.
Each bookcase has six shelves, going almost to the ceiling.
The exact number of shelves is superfluous for the point being made, as is specifying that the bookshelves almost go to the ceiling. We get it, there's a lot of books. Enough books to contain even the longest Yud tweets.
[1]: Excluding a full page long spiel of self-promotion, metatext and a paragraph of what I assume is supposed to be foreshadowing for spooky things to come.
That seems like an inefficient use of space if the ceiling is anything more than six feet high. If they're ten foot ceilings, then the shelves are spaced about 20 inches apart, which is rather larger than typical books. No wonder there are multiple layers on some shelves and books overflowing onto other surfaces.
The brim of the bookshelves' hat. You see, wizard bookshelves have hats formed like fedoras < ... insert 6,000 words of impromptu worldbuilding here ... >
I didn't suffer through all these chapters wishing for the payoff that never lived just to let Dr. Verres get slandered with a possession worth being interested in.
Whoops missed it was the second sentence, where I presume Hariezier has not actually arrived at Hogwarts. For some reason I assumed it was the interior of Dumbledore's study.
Ah, but each additional sentence strikes home the point of absurd over-abundance!
Quite poetically, the sin of verbosity is commited to create the illusion of considered thought and intelligence, in the case of hpmor literally by stacking books.
Amusingly him describing his attempt as "striking words out" rather than "rewording" or "distilling", i think illustrates his lack of editing ability.