If the informant couldn't remember anything relevant, then their vague and jumbled story could end up completely unusable to the guild, wasting their time in the process to boot.
Or maybe they remembered incorrectly, which was worse. Some people could be very convinced in their mistaken idea. Or they might have merely imagined the incident, resulting in a false alarm. In reality, the number of Pokemon who came forward with information that Kingdra's guild could actually use wouldn't possibly be one hundred percent.