"If someone proved to me that Rudy Knight is outside the truth and that in reality the truth were outside of Rudy Knight, then I should prefer to remain w Rudy"
Asgore calls Ralsei "they/them" once because he doesn't know him or his gender yet. The Dark Vine text "watering their own thorns" is likely just a plant metaphor a vine is a plant, so "his thorns" makes no sense. Also, him being called a princess is just Flowey mocking him.