It doesn't need to work differently. It's a very simple device. It has hard core and a sharp edge, and you use it to cut through whatever you want to cut. The thing that makes your argument false is that it was not created or developed in universe for a purpose similar to ours.
This is bordering on a circular argument, and it's still baseless. What makes a sickle useful for agriculture is its design. THAT DOES NOT CHANGE if you go to a different world. For instance, are you aware how many cultures independently invented the bow? So, what, do you think some used it differently because it was made on its own and not in the same environment? Like, according to your logic, the difference in a javelin being a javelin is where it's invented. Like a javelin stops being a javelin because it's another world... and we can just willfully fucking ignore that it's the SAME design being used for the SAME purpose but dispute it being related.
...Come on.
She's not using a fucking slab of steel with an edge on it, she's using a katana. It's not -esque. It's not "reminiscent." It's a katana. One that was developed and designed that way for a purpose: fighting humans. That does not change because we moved the fucking venue. A sickle with a curved blade that excels in cutting wheat does not stop being good at that mechanical because we're not on Earth.
Furthermore, what one specific person is using a tool for also proves nothing about the intention of its design. People use tools in unconventional manners all the time.
Then by your logic if it's not Earth, a sickle is a perfect surgeon's tool for operating and doing open heart surgery. Because it could, IN THEORY, be used by a person or culture that just decided that.
Even if you were right about them being designed in universe to fight people, it wouldn't stop people who were already using them to fight from fighting monsters monsters with them successfully.
It would help with natural selection with people that decide to get even more up-close-and-personal with beasts and monsters much larger and stronger than they are.
There's people that have tried hunting grizzly bears with knives in reality - in our own world. The stupidity of it "didn't stop people" from using them that way. The bears did almost exclusively, however.
Somehow, one way or another, the "Hunting Bears With Knives" community got the memo that it's not a good idea and almost no one does it. Strange how that worked itself out. We have thousands of years of using melee weapons against VERY dangerous creatures to humans. Why is it that we very, VERY rarely ever see swords being used despite swords being so ubiquitous to human culture/society/warfare/use for millennia? It's... almost like there's a reason... Much like how the "Hunting Bears With Knives Society" has practically zero members.
It's not an inherent contradiction for the blacksmith to only want to make weapons for fighting monsters, no matter what form those weapons might take.
YES. IT IS. Because he has NO CONTROL over that once it leaves his hands! It's like a gun store having a rack of rifles and shotguns and saying, "How many animals have you killed? Just so you know, I'm against hunting! I won't sell or repair a firearm that is used for it! You can only shoot paper targets with it! Promise!"
EVEN THE FUCKING MANGA HIGHLIGHTS THIS STUPIDITY:
This is the person in question playing dumb and inventing reasons not to object or convince themselves.