A court found her guilty of knowingly being an accessory to mass murder, so a court decided she did know there were crimes going on, and that she knowingly assisted in those crimes.
Stutthof Concentration Camp was a tiny camp where 63,000 to 65,000 people were murdered over 6 years. There were 110,000 inmates who were mistreated constantly. It is not really possible that she missed all this.
Furchner intentionally surrounded herself by people being tortured to death, and materially helped the system that tortured them to death. She was not forced to help that system, but instead had to actively work to be part of that system. She did it for the rewards of stealing stuff from innocent people, the social promotion that being part of genocide brought her, and because she really believed that genocide was a good thing.
A few years back, a Black man was executed in Texas, not for killing anyone, but for driving a thief to a place where the thief mugged someone and shot someone when the mugging went bad. That was one victim murdered, but even that one murder was not known ahead of time.
Furchner woke up every day knowing she would be part of several murders that day. She then helped organize those murders. She worked hard to insure those murders.