Awesome run here, it's done in a much more straightforward manner than is typical but lots and lots of happy intelligence from various players that somehow makes it feel a bit more special. The humans were not in fact entirely fooled by the demons at all, they actually gave the entire situation a bit of thought and put two and two together to get four for once, though they still underestimated their opponents (or at least it appears to be that way but maybe that too will turn out to just be a front). The demons in turn didn't hesitate for a second to change tacks.
Fern and Stark though are still showing their relatively lack of experience. Both have dealt with fairly strong foes by this point, but aren't battle hardened yet either. They discerned correct that the lead demon's attention was entirely on Frieren, but incorrectly assumed that was because he was so skilled/powerful he considered them not worth considering rather than that it's simply Frieren has history and reacted in a unique way, and he was reacting to that not a sense of power.
Already can't wait for the next chapter!
Edit: Also, I'd forgotten Chapter 2 Page 10 from back when Frieren first met Fern:
She truly is near undetectable to mana sensory
I don't think it's really come up again since then has it? But that could be a huge advantage vs demons if they depend heavily on magic sensing to determine threat level. Or for that matter even if they depend on reactions because Fern is such a poker face overall.