The problem is likely due to uiop:launch-program (the function we use for external editor invokations) accepting only a list of strings. I’ve just pushed a fix on master, now a mere string should be fine too.
By the way, why init.el instead of init.lisp? Nyxt is written in Common Lisp, not Emacs Lisp.
@fenix Note that specifying the program as a list of strings is handy since you can also declare arguments. For instance, here’s how I set external-editor-program: