It's all the more tragic how it's becoming clear that the demon king isn't some ultimate evil in this world that the human side in these types of stories tend to paint him. Demonkind itself in this story seems like just another race of people and just another kingdom in the world. Sad, knowing the human king is evil and corrupt. But it would be delicious irony once the daughter of the demon king becomes adopted by the very hero he sent to die fighting the demon king, and then they all live happily in the land he himself maliciously granted to that same hero.
Duke's desire to stop his daughter from succeeding him as demon lord lined up with my suspicion that he did something to the armor so Alice could not use it, ensuring she could not become the next demon king and/or become a tool of war. But it didn't turn out to be the case and this chapter debunks it. She just didn't fulfill whatever requirements it had to use it until now.
So it wasn't merely "armor" after all. It's a cool versatile accessory that can even be a weapon depending on the user. Normally I'd complain why her chest isn't covered by "armor", but this thing is technically in "weapon mode" so the partial armoring can be forgiven. Maybe Alice wouldn't need to be rescued that badly after all, seems she's gotten pretty powerful herself. But we'll need another way to get her to join the family if rescue is not the way.