I can see different motives at play here.

Mrs. Smith felt sure that Anne was going to marry Mr. Elliot. Everything seemed to point to it. She was puzzled as to why Anne seemed to deny it so strenuously. By encouraging Mr. Elliot's suit, she was testing the waters, so to speak. She was trying to gauge what was in Anne's heart- was Anne merely being reserved with her, or was Anne really determined to not marry Mr.E?

Perhaps, when pressed, if Anne had said something more positive about considering marriage with Mr. Elliot, Mrs. Smith may not have revealed anything further about her knowledge of his real character. After all, a woman in her condition may be pardoned for showing some self-interest. Besides, Mr. Elliot may well have treated Anne better than he did his first wife- he did seem to recognize and appreciate her worth. After all, Col. Brandon in S&S did not choose to enlighten Elinor/Marianne about Willoughby's true character before the latter's engagement to Miss Grey. Willoughby was a real cad, compared with Mr. Elliot, who, at best can be said to be selfish, disloyal and a hypocrite. He hadn't seduced and forsaken pregnant teenage girls like Willoughby did!