Now that I read the comments (most of them), my two cents.
I approach this issue by another angle.
Yes, it is simply wrong how they all react to Shizuko's "inventions", it's wrong, no question here.
But here is the thing, even if they know about other or better weapons don't mean that they're desirable weapons. Like it was already discussed here, defying common sense, firearms was easier to produce at that time, at that place.
To begging with, the primary weapon was not the katana, it was the spear.
It's efficient and it's cheap.
The country being fragmented means a lot of small and poor, resource starved, states. They couldn't even produce decent katana, or good quality bows by the lack of decent materials. Spear was the way to good, and they went. Lords of those times left records about teaching their sons/successors to invest on spears.
Muskets raises questions about how to obtain or produce black powder, I don't know how they did, but the gun itself, it could be produced with low quality metal. It was possible to produce so many so fast because there was very well established centers of metal work. Almost all those weapons came from the same few places. It was just a quesition os placing an order for muskets than bells, for example.
By anyway, this author don't really care, right?
He just want to have some fun.
(but don't stop with the discussions that they're good read).
Up on the official site you can read the most recent chapter.
Look how kakoi Shizuko is:
@Mogima
"nurturing" makes it sound way creeper.