...but also, UA and ANA have a JV. What's the point of this flight? "United already has a flight on this route" that's operated by NH.
Because HND is the shiny new toy/slot pair that every airline wants to have (and to deny to another competitor)
If UA gets this slot pair, UA can only fly IAH-HND.
And most likely NH will move their existing IAH-HND slot pair to something else that UA+NH prefers instead... And whatever NH does with their HND slot pair is NOT subject to USA DOT oversight. So basically UA-NH get to choose what they really want (3x jfk-hnd or whatever else