"Side-piece" as a concept has existed for a while. I don't think coining that term has any meaning; people have been compartmentalizing their dalliances since the dawn of time
I'm not so sure I'd be so quick to dismiss the differences here. Another example is the notion of having a "finsta" vs their normal Instagram. The finsta is a private Instagram they only show to real friends, while their normal Instagram account is for the world.
This isn't merely "compartmentalizing their dalliances," it's compartmentalization of public vs private (or at least quasi-private) self.
