In this quote, Rosmarin (and many other critics as well) is stating that Jane Austen's Emma is deliberately designed for the purpose of tricking the reader.
This novel is famous for misleading the reader! Austen does this by setting up Emma Woodhouse's consciousness as authoritative, but at the same, making Emma clueless, as the movie Clueless, based on the novel, points to in its title. We see the events that unfold through Emma's eyes, but Emma lacks vital pieces of information and also arranges the world as she wants it to be. Almost everything she comes into contact with she manages to misinterpret.
In this way, she is not so different from most people, as all of us tend to perceive the world in a way flattering to ourselves. Therefore, despite hints, Emma completely misses that Jane and Frank are in love. It is more flattering to her—and consistent with how she has been raised—that she would take for granted that Frank is courting her, not using his flirtation with her as a cover to divert people's attention from his real love interest. And because Frank as her suitor is how Emma, with all her supreme self-assurance, interprets events, we as readers simply follow along and are as surprised as she is when his secret engagement with Jane is revealed.
Like Emma, all of Highbury is tricked—as we all tend to be the center of our own universes—for it seems entirely natural to them that Frank would want to come and spend time in Highbury visiting his father and new stepmother—even though he never has visited before and didn't attend the wedding. It never dawns on them that Jane's extended visit home is the one and only draw.
Austen loved a good joke, and in this novel, she enjoys having fun at the reader's expense—as Rosmarin notes.