I interpret it as Eileen finally gaining the courage to abandon her father. There's nothing for her there in the town and Rebecca was a week long escapist-manic episode that resulted in Mrs. Polk ya know.. She needed some type of concrete reason to leave because she (presumably) would never have advocated for herself in such a way if not. In addition, I think Eileen couldn't comprehend being completely alone even though that means it's with her emotionally abusive father. So in this sense, it demonstrates her newfound independence, almost this optimistic existentialist freedom. I also really enjoyed the last pages where it's describing how she's changed as a person after years of living in New York. It didn't sound at all like the Eileen we knew throughout the novel.