The obvious answer is that these aren't the Eld at all. They are a group of trickster faeries who have been eavesdropping on the group. Steffanie's spell was a total failure, but the faeries created the illusion of it succeeding. All the information that they "magically" know is just stuff they overheard. Those expressions in the last panel? "Oh, crap, he called our bluff! Think of some way to spin this, quick!"