This couple actually talks about things. The talk matters. That is it is not just fodder for the relationship. It drives the plot as well as the character development. Like real life. It works so well they don't even need a sex scene. Of course, this is Hollywood, they do have one. Yet it is clear the movie would be better without it and their relationship would be better without it.
Still it gets very close to a Catholic understanding of what a sexual relationship should be. The concept of "Until death do us part" is very much present. They are both dealing with cancer and not really up for anything frivolous. They want to know if the other person is truly willing to walk through some serious pain with them. In fact, Disneyland is mocked as a hopelessly unserious thing.
It reminded my of Angela's Ashes where Frank McCourt falls in love with a woman who was not expected to live long. There is sex and they are not married but you wonder. They are giving themselves for as long as they both shall live. He says he never loved any woman as much. You wonder if the church should expedite marriages like that. People wanting to walk through such suffering together maybe don't need the normal marriage prep course.
Jesus does not just say love is stronger. He proves it. He died and rose again to show us exactly that. The other thing is Jesus does not just say it as a matter of fact. He says we need to believe it. We need to have faith in God and believe in the resurrection and live a life of love in the face of death and suffering. This movie does not go there. Christianity is not just an academic exercise in figuring out what is true but a challenge to embrace Jesus as the Truth.
So it is not really a conflict between nihilism and Christianity but rather between nihilism and a pseudo-Christian romanticism. It does not really consider the fullness of Christianity. It is a bit like Moral Therapeutic Deism but not quite. They have suffered to much to believe the therapeutic part. So do they believe any of it? That is all that remains. Embracing all of it is not even really on the table.