The title of Saki's short story "Dusk" can be justified in that criminals usually do their work in darkness or near darkness as in "Dusk." Criminals are rarely seen doing criminal activity in the daylight. They want to protect their identities; therefore, they come out at dark.
In this short story, Gortsby is correct in that people who are defeated or dejected often show up at dusk. They too are protecting their identities. Many are ashamed of their failures and do not desire to be recognized.
Dusk is a perfect time to meander about without fear of recognition. All people have to come out at some point. It makes sense that those who fear being recognized for their failures would prefer to come out when it is nearly dark. Since most people have to come out sooner or later, waiting until it is nearly dark helps conceal their identities. No one will hassle them about their defeats or failures. Hopefully, no one will recognize them.
Saki wrote about the criminal attitudes and behaviors of man. He clearly understood human behavior. He realized that criminals or those who are just defeated would tend to protect themselves from others:
Many of his brilliantly crafted, deeply sarcastic pieces, however, deal with the criminal impulse of man.
At dusk, people appear as shadows. One shadow turns into another shadow. No doubt, the short story's title is justified. Near dark is a perfect time to come out without recognition.