A Japanese studio makes a game where it's extremely difficult to learn a katana specific swordsmanship skill. That doesn't sound overly probable. Not to mention you can still use a katana with regular long sword skills. Katana is nothing but a curved long sword. Maybe it's not 100% efficient, but it would be pretty weird if, after a while of using it, the katana skill itself didn't appear. The author is just writing whatever here for the sake of the (nearly nonexistent) plot.
In case someone forgot, Crim learned an axe skill by chopping wood to repair her house. So, it's already in the game engine.