This statement comes a few paragraphs from the end of Eagleton's essay, when he has been pointing out what an endlessly diverse form the novel is and how difficult it is to define as a form (which is why there are endless candidates for the position of "first novel" in the world, and in the literature of every country).
The essay discusses the unusually large role of women in writing novels, suggesting that the finely detailed observation of a Jane Austen or a George Eliot was produced by women's need to understand a potentially hostile world dominated by men. Eagleton says of such female novelists:
They were spontaneous semioticians, who needed for their own sake to be skilled in deciphering signs of power, symptoms of dissent, and fruitful or dangerous areas of ambiguity. All this lent itself to the writing of fiction, even if the same set of talents lends itself to being a successful tyrant.
It is in this context that Eagleton introduces his paradox that "the novel fostered a resistance to authority at the very time that it was becoming a resourceful medium of middle-class cultural power." The skills of a fiction-writer are the skills of a tyrant in the same sense that the fiction she writes both resists and reinforces authority.
The novel, Eagleton contends, "became the supreme arbitrator, in the sphere of cultural representations, of what was to count as real in the first place." In this sense, what was not represented in novels was as important as what was there. The novel allowed and even encouraged social criticism, but first it defined, perhaps quite narrowly, the society that was available to be criticized. This is how it came to embrace subversion and authority at the same time.