That's more of a plot convenience, imo. But keep in mind that they live in a world where war is the status quo, people have various Odo levels and superhuman beings like Heroes exist. It's shown that the Hero that the MC saw when he was a novice mercenary already overpowered him significantly in term of martial prowess and talent.Really? Girl shows up, “I’m an innocent kid”, “innocent kid? She can’t be a spy, Let’s take her in”, girl uses sword and trounces an adult professional soldier, no one says anything except as an after thought?
From how I look at it, the idea that a little girl from the boonies who's been drifting in the battlefields scarvenging stuff for survival might have been trained in or forced to learn how to fight turns out to be talented enough to beat the garrison's own talented young soldier in a duel might be a rare but probable occurrence in their world.