I think your assumption this is meant to be in the 1900s is erroneous. Considering very little else matches that, instead most things seem to be 1800s era, surely the more likely answer would be the author just didn't know it wasn't invented yet?Alright, I refuse to believe that they don't know about christmas. This is at least after 1949 since they ate currywurst, they already went through a winter and there's 0 way that Sieglinde didn't already celebrate it, and they aren't exactly isolated from the rest of the world since they have supply lines enough to get coffee and chocolate.