“This meaningless course he put me on” is pretty fancy language, for Harry.
It makes sense that Harry's vocabulary is rather idiosyncratic. Ze was barely able to read before entering Hogwarts, but then started reading lots of wizard books (using Luna's spell). And wizard books tend to be... sesquipedalian.
Approximate readability: 6.92 (244 characters, 49 words, 5 sentences, 4.98 characters per word, 9.80 words per sentence)