It is not about that "Isten védjen" is in any way wrong gramatically or that you cannot say it. Hurray for the Hungarian language, you can practically say anything.
I just have never heard anybody saying that to another person in my short 31 years of life
Which of course does not make the expression invalid and even Jókai had used it. (That is why I translated it, it was translatable...) Though literary context can be deferent from the living language, especially, if it is 150 years old.