Lady R describes herself as being "no matchmaker". She tries to persuade where she thinks someone needs a little guidance, but she lets people make their own matches. Since LR has eyes in her head, she can see that Mr. Elliot likes Anne. She is inclined to approve of his choice. What sane person would object to him choosing Anne over Elizabeth.

Lady R knows her limitations. I respect that. Why should she try and encourage Mr. Elliot to choose Elizabeth? She has no influence over him. It is possible to hint to Elliot that Anne is not inclined towards him and see what that does... But Lady Russell sincerely hopes that Anne will marry Mr. Elliot.

But Lady Russell would have been happy if Mr. Elliot had stayed by Elizabeth. (Though she might have thought less of him).