I don't think this kne fits the theme of this set of stories. I get that technically, yes, they end up together. They're married to the prince though. That's poly, not yuri. To fit this series, he should have just broke off their engagement.
I feel like the artist actually read this and saw how much they screwed the prince over, who seems to be a good guy. I feel like this ending is just their way of saying that they didn't know how to end this without completely throwing him under the bus and setting it on fire, so they did this.