Since why their scouting of the dragon failed has already been well-explained, I will probably just add that if readers have any doubts as to any details in the story, it's probably much clearer and/or will be explained later. The WN is very meticulous about detail.
And I should add that it when it comes to the actions of people, it takes into account the different motives, levels of expertise, cultural differences and biases of the characters. It may be easy to pontificate about why X is illogical or why a character should have thought about Y, but it usually makes sense in context.