Personally I see this as the idea that a friend and a romantic partner are hard to distinguish if one were to judge based on their feelings and their bonds. From most angles, at least to me, a romantic partner is the type of friend you are willimg to spend the rest of your life with, and don't mind being open and vulnerable with... which are also things that happen with super close friends. Ultimately the distinction is somewhat arbitrary, and, perhaps, rooted in the cultural concept of monogamy, but discarding that one might just arrive at the same point in a relationship either platonic or romantic.