It was not unheard of for critics to write very severe things about books they had not read (nor was it unheard of for critics to write very flattering things about books they had not read, but had been paid well to write about.)

The Quarterly were 'friends' of Jane Austen: her publisher bankrolled them, and William Gifford (he of the perfectly placed semicolon) if not Walter Scott, personally enjoyed Austen's work (Scott was unambiguously a fan of Austen's when he knew she was dead, but I suspect he saw realism and mysterious authorial identity in anyone but himself, as unwelcome competition, and would rather promote abject imitators of his self than an author with the power to make his works look like melodramatic rubbish. You can see his influence not only in the opinions of contemporaries like Edgeworth, but also on later Romantics like the Brontes.)
In any case, the Quarterly (and later, in Scotland, Blackwoods) was a Tory paper, set up with the express purpose of opposing the critics of 'the yellow terror' - the Edinburgh Review, a Whig periodical.

In 1818 in the Quarterly, Gifford had claimed "It is not that Mr. Keats (if that be his real name, for we almost doubt that any man in his senses would put his real name to such a rhapsody) -- it is not, we say, that the author has not powers of language, rays of fancy, and gleams of genius. He has all these; but he is unhappily a disciple of the new school of what has been somewhere called 'Cockney Poetry,' which may be defined to consist of the most incongruous ideas in the most uncouth language."

The same year, Scott's son-in-law, John Lockhart, in Blackwoods, about the same poet and poem "To witness the disease of any human understanding, however feeble, is distressing; but the spectacle of an able mind reduced to a state of insanity is of course ten times more afflicting. It is with such sorrow as this that we have contemplated the case of Mr John Keats. ... He was bound apprentice some years ago to a worthy apothecary in town. But all has been undone by a sudden attack of the malady. ... For some time we were in hopes, that he might get off with a violent fit or two; but of late the symptoms are terrible. The phrenzy of the "Poems" was bad enough in its way; but it did not alarm us half so seriously as the calm, settled, imperturbable drivelling idiocy of Endymion."

The Edinburgh Review were near as bad, but of the opposite political persuasion. They wrote some equally cruel and ignorant rubbish about Wordsworth, Coleridge and Byron (who had all turned from the Whig to the Tory cause around the same time, and for the first two, if not all three, at least partly because they were paid to).

"The Cockney School" the reviewers so loathed was a reference to Leigh Hunt. His short-lived periodical The Examiner, was the first experiment in Independent Journalism (ie. the journal was not owned by a politician for the purpose of promoting his cause, did not accept money from the crown in secret for promoting its causes, and endeavoured to report on the political, theatrical, social and literary events of the day truthfully and without bias, or at least without paid bias.) Of course, Hunt's crew were hated by both sides of the absolutely rotten body politic, and pilloried by every other press.

In any case, when you read some stupidity of this sort (and if you look at what the reviewers have to say about any female author, especially those who are not wealthy and titled, whose works have only the merit of being better than those whose works they are compared with - eg. Anne Radcliffe/ C.R.Maturin, Jane Austen/Maria Edgeworth, you will see it often), it is quite possible the critic has not read the work, or has not been paid enough to give a good opinion of the work.
And when you consider, the author was dead, but and in Northanger Abbey she openly attacks the critics. The critic might not want to acknowledge that aspect of the book, but can't afford to entirely ignore it either. On the other hand, Persuasion is an implied critique of those two great pillars of the Tory cause, Patronage and Noble birth. Worse, Sir Walter could be a portrait of another Well-looking man of the same age, graceful but in debt. There seems to be a case made in the novel that the welfare of the country could be better served if it were in other hands. Add to that, it seemingly promotes the right of women to marry whom they chose, and the merits of wives being as intellectually and actively involved in the economic aspects of life as their husbands (to the point even of hinting that some wives might be intellectually, morally and in practical matters superior to some types of husband). Maybe the easiest way out was to avoid much comment, and simply declare the work immoral (because it would have been so much more moral for Anne to marry Walter Elliot, and Henrietta Captain Wentworth!). And it is not without its own fling at the fashionable Romantic sensibility this reviewer hypocritically criticises when it comes to heroines choosing love over money, but derides when it comes to authors writing naturalistically rather than in artificial passions.(although, if you notice, the following review is a wonderfully hypocritical piece upon Byron's Beppo, so it is hard to say what could please this critic. Maybe he had no taste for satire.)

These critics did a good job promoting Rob Roy and the Rackrents, turning Jacobites and Republicans into the fictional Loyalists and best-sellers of their day, using Establishment politics and religion to define how the poet ought to feel and what the novel ought to elevate, but Where are those songs of spring?

Those foolish critics are long dead and would be forgotten but