That'll require opening a can of worms leading to more complicated scenarios injected into the currently varied SOP. You'll need to involve safety inspectors, meteorological experts and a host of lawyers to determine that. May lead to global change of the definitions of "twilight", "VMC" and "safety". Thus, making everything as clear as mud!
Guess if the airline had already implemented RNP procedures, this incident and its associated negative publicity would have been averted.