That's not a "gentle" slope when it comes to building on it.
Unlikely a relic would exist from 200 years ago? Did you expect it to be stored in acid or something?
But that's what she was going to do with that snowball...
It looks like they changed Cayna's lines for the manga, which made her anger more confusing for anyone that hasn't been taking detailed notes.
The clues were all there, and I think it worked well in the manga. First she gets angry without any obvious reason, and wants to go to a regular inn. Then she says he insulted something important. The second part gives a hint about what the subject was, and the last about why she got angry. Short term foreshadowing with a fast payoff, which highlights that it's not supposed to be super obvious in the first place.
I checked the raws, and she says she is 17 years old. Seems to be an error somewhere in the translation process.
But she suddenly appeared about 200 years in the future, so her age gets complicated.
It's not that complicated, really. To her, she's 17, and she's existed in that world for a year. To the others in the world, she's an old hag of some respect.
But her reaction doesn't make sense since she's already well aware of it, and has met the other kids before. It shouldn't come as such a shock to her at this point in the story.