Of course, the quote you have cited could be argued to be the main message of this excellent story that uses situational irony so well in its ending. Note how the story is told using the third person limited point of view, which means we see everything from Norman Gortsby's perspective, only having access to his thoughts and feelings. Thus it is that we can see how the young man appears to him as he is described to us. Gortsby of course exposes the young man's deceit, but then, after the young man has left the scene, finds a bar of soap by the bench where the young man was sitting. Ashamed of his judgement and attitude towards the young man, who, Gortsby believes, has his story proven by this object, he finds the young man, returns the soap and gives him some money. As he walks back to his bench, he thinks to himself:
It's a lesson to me not to be too clear in judging by circumstances.
Of course, as he goes back to his bench only to be met by the old man who was sitting next to him at the start of the story looking for his bar of soap we see that external circumstances can be incredibly deceptive, and that Gortsby was actually correct about the young man. He was trying to deceive Gortsby as a confidence trickster, but the "lucky" find of the bar of soap ironically supported the young man's story, showing that Gortsby should be "clear" in trusting his intuition and that external circumstances are not always what they seem.