I don't think us fighting Stella in the streets of Insomnia is a rumor. Spoilers:
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Seeing as how we fight Ardyn in the streets of Insomnia. And in his second phase, we fight him with magenta/pink Armigers. I don't think it's a coincidence. Seeing as how they could have colored his Armigers dark like the Starscourge, and the fact that they cut his Astral Chaos form, and not his Armiger phase.
And while I do agree Luna wasn't as involved as Stella in the story, I'd say that, if given more time, Luna would have had a bigger role.