About this episode...
The author actually made alternative world chapters of "what if Rem DID agree to run off with Subaru?"
They actually live happily ever after and have two children.
He posted these happy chapters during time of despair in the later arc, making it all the more "hey let's rub salt in the wounds".
They're basically spin-off chapters and have no relation to the canon story, so I doubt it'll come up in the anime.