] MS is Anne's former governess, and Anne has not seen her in years. And Anne spouts off like this?

Mrs Smith is not Anne's former governess - Anne became reacquainted with her through her old governess:

"She had called on her former governess, and had heard from her of there being an old school-fellow in Bath, who had the two strong claims on her attention of past kindness and present suffering. Miss Hamilton, now Mrs. Smith,..." (ch 17)

I think Anne is getting fed up and frustrated with the assumption that she is going to marry Mr Elliot. Lady Russell has been going on about it, rumors had reached Captain Wentworth (his friends in Molland's talked about it) so they may have reached her ears as well. And the previous night, she has just worked out that Frederick still loves her, but is jealous of Mr Elliot:

"How was the truth to reach him? How ... would he ever learn her real sentiments? It was misery to think of Mr. Elliot's attentions. Their evil was incalculable." (ch 20)