He seems serious. He mentioned that she won't be free from this relation until one of them dies😆
Both of them will be in this relationship for life. That's a big hint, symbolism, that's been given by this dailouge.
Also if you guys remember, earlier Riddhima was wondering that she had written Kabir's address in the cab, but she reached VR mansion. Her destiny is Vansh, she can't change it.
Kabir will be out of the picture as the story progresses. Riddhima is a very sensitive and emphatic person, maybe she will subconsciously start caring for vansh, when slowly she gets I have to know the incidents of his past.