I don't think it makes sense, either. I think it could happen, perhaps. My son passed a food challenge and then reacted to small, not trace, amounts of the food and now has a lower threshold for the food. Very weird. His threshold level changed wildly in the past 7 years since he had ana to that food, went on the pass a challenge, reacted to tiny amounts, and now can eat more than the tiny amounts that cased that reaction. We have been told and tend to think that we either grow into or grow out of allergies but my son shows it isn't always a straight line or always that simple.
But in any case I do not think there is any way they can say with certainty that the child reacted to trace peanut. How can they know it was for sure PEANUT. A lot of peanut allergic kids are also allergic to one or more tree nuts. There are some tree nuts that are not commonly used by manufacturers. I know of several kids who didn't find out they were allergic to a more rarely used nut until they were much older because they just hand't ever eaten something with that particular tree nut in it. So, I will sit undecided on that story for now.