I dont disagree the polishing of language is not a prime element of novels or romances. I doubt Stlukes would, as he know so well Borges and his critic towards Cervantes, exactly due to that. I do not think it is even the prime of poems (after the is poems and romances together), but I won't deny that a Novel gets better if the prime elements where better treated by the writer. Now, Dostoievisky is no Proust or Flaubert, but he is hardly dense. Brothers K advantage of Madame Bovary is that the language is better treated in action with the characters, Madame B advantage is that language is better treated as structure. In this; neither are perfect (and both prime suspects of greatest novel ever. Nobody calling it would be insane.).
When Borges got older he gave up the claim Quevedo would write a Better quixote. He said something like "That was the only Quixote possible". Nabokov is attacking Dostoievisky style (and with some reason) but frankly, those things do not move a single line from Brother K towards perfect. It is close as it could be. As some point the second part of Lolita is poorer, but what they do not see, the second part had to be that way for the first Part make any sense. Nakobok was a prisioner of his style. It had to be that way.
I would lay to say the greatest stylist of Russian literature is Chekhov and nobody else. Seems to me that he had to write short stories to allow his style to not get those flaw that a romance or novel would bring to it, when people would think his style was superior to his novel. His solution to combine Dostoievisky and Tolstoy was this one. Not fighting with them, being something anew. And fully aware, style is substance.