I couldn't have phrased it better myself @AllProperTeaIsTheft!
I had a browse through that critic's reviews. Almost without exception, his reviews are of serious pieces of literature. And he apparently works very hard to use the maximum amount of words to express any single thought.
His review of a children's book seems bizarrely out of place - especially when he confesses that he's writing a damning review of a book that hasn't actually been released yet, and that he's not read.
He provides scant examples of why the writing is so poor in his opinion, other than to be scathing about JKR's use of adverbs. I wonder if he's forgotten who the target audience is here? I fail to see anything wrong with her use of adverb, especially as it makes the dialogue much clearer for children.
As for other vague accusations such as plodding text, surely that's very subjective? I didn't find it to be plodding, nor overly descriptive. What I did find to be plodding and overly descriptive were his reviews. Not just the HP one, but all of his others. He seems determined to cram together a sequence of words that he thinks expresses his superior knowledge of the English language. In reality, his reviews are ponderous, lack detail, and often veer from the subject. I find him to be self-indulgent, and extremely pleased with himself - but I also accept that it's a subjective opinion that others might not share.
I enjoyed JKR's books and I disagree with a PP - you can't make a silk purse out of a sow's ear. Stephen Fry may be a marvellous orator, but if the writing was poor, the audio book would still be dire.
It doesn't really matter to me if a book is "written well" if I enjoyed it. But I was curious to find out what I had evidently missed, and so far there's nothing to objectively show that the writing is poor. It doesn't matter to me either way whether JKR's writing is good or not, I don't have any reason to defend her. But I can't see any of the accusations of poor writing have any substance other than subjective opinion.