Many things that are water-tight are not water-vapor tight.
Furthermore, the moment you opened the case, the humidity inside would match the humidity outside. Even if it was sealed in such a way as to prevent water vapor from getting through the seals, it would only work as a way to preserve the guitar without playing it if you put it in the guitar when the humidity was at an okay place, and then didn't touch it again until the humidity returned to an okay place.
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