This comic has nothing to do with Twilight becoming an Alicorn, but rather realizing the full potential of her magical ability - which in this comic is joining the two Sister's as immortal sky goddesses. And I've always liked the Sisters as goddesses; I think you're being too stringent with your definitions. Who but a god can raise the sun and the moon? Demigods, in greek mythology at least, were LESS powerful than the Alicorns are. They were strong, beautiful and favored by the gods --- Celestia raises.the sun every day, like Helios. Also it's just fanart, try not to take it so seriously
This is based on Romana's actual regeneration scene way back in the Baker era, where they literally hired an actress who had played a character in the previous episode and Romana CHOOSE to use her likeness for her new regeneration. Obviously this has been grosllsly simplified for a 3 panel comic but yes, this takes place in the Tardis control room. Also Romana has just announced herself, because, as I recall, the Doctor was indeed confused as to how "Colgate" got on board.
Dunno if this scene is available on line anywhere, but it was pretty good.