Do they really know that it was all her doing it?
Besides, it's still just a little unarmed girl in front of them.
Depends on what you mean by "all of it". She herself admited to designing and building the catapult, a weapon that didn't exist in this world (though the MC already presented the idea). She calls herself an "engineer" when no one so far in the story used such a term (and indeed there would have been craftsmen instead). She also claims to be from a forest on the country's border, which sounds familiar.
Is she also responsible for iron technology? Can't say but it's logical to assume that, after all the concept is more involved than just the ore itself. I suppose there could be a twist in which she's just another McGyver girl like Valetta (though the latter at least has the excuse of having access to otherworldy knowledge from Kazura's books), but that would mean the enemy country has someone else that actually is an otherworlder - even their general noticed that it's just weird for the names for both countries' innovations to be the same. He probably assumend Arcadia stole it from them, since he obviously does not know about otherworlders, otherwise it would have clicked to him immediately.
As for her being "just a little girl", I get that, but consider one thing - she had no problem providing the military a weapon that crushed the fort's walls and resulted in much bloodshed on both sides of the battle (for all the survivors they let go, there were still said to be a lot of casualties). She was quite proud of herself, in fact. This is in stark contrast to Kazura, who was reluctant to share any offensive technology until this very incident. The only reason he did was to save someone he cared about.