I have my doubts on people actually knowing off the top of their head how to make soap. I hate to be ageist, but I think people saying this are younger readers. You'd be surprised how much information you push out of your head when you go into the working world. I can program and build computers, but I sure as hell don't have a clue how to make soap. lol It simply has nothing to do with my job nor my interests.
Only thing that annoys me is that they seem to have a fridge (hopefully I'm wrong and it's some kind of alternative) and if so that's a huge oversight.
This is not the only LN-based manga that has made this mistake though. Like The Bookworm ( Honzuki no Gekokujou) Movable Type was around in the 1400s, yet they have glass windows in that manga (1700s). You're telling me nobles and commoners have a 300 year gap in technology when they are literally down the street from each other?
You've just got to let it go. Some authors are simply and honestly just...mistaken.