It's a mistranslated line, and has different characters in the scene.
Basically, here is what you need to know.
There is an idion is use: The original idiom is thus: 毒にも薬にもならない 。AKA “Neither poison nor medicine”. This idiom basically means “neither doing harm nor good” or “ you can get nothing useful out of it “. It is neither a poison or a medicine, so it is not doing harm nor good.
However, what Edelgard says is 薬どころか毒にさえならぬ, which twists that idiom a bit. The original translation is a pretty close an English equivalent translation. “Far from a medicine, not even a poison” would be a bit more literal in the meaning. So, the full line would be “Even if you let it sleep where it is, far from a medicine it’s not even a poison.”
But the ultimate meaning of what Edelgard is saying is:
Without the previous line to her idiom, this may seem kind of strange, so here is the full translation (done by me, the idiot, so it may be a bit off):
So, yes. She’s basically confirming that Claude was correct in his guess; that she’s there to ‘reveal’ the sleeping treasure. That she plans to take them, and that they are useless here just ‘sleeping’.
And since Claude is in this scene, unlike in CF, the line is different.
You should keep playing if you haven't completed the route before you get answers for this.
Gameplay.
Idk. Gameplay I guess