Here's a look at some of the lyrics: "And once again / I’m really glad you’re my friend / We could do it all again / It doesn’t mean it’s the end." Since Sally added it at the end, I'm guessing the author's addition to the playlist means she was thinking this is how Marianne felt about Connell's important decision at the end of the book and first season of the show.
When I found this, I emailed Greta Kline (Frankie Cosmos) to share my excitement and she said she's a fan of the book, so now I love this inclusion even more.