When Norman Gortsby declares that dusk is the "hour of the defeated" he is indeed being his typical, blindly-judgemental self. In his obviously limited opinion, dusk is a time for miserable people to show up.
Men and women, who had fought and lost, who hid their fallen fortunes and dead hopes as far as possible...came forth in this hour of gloaming, when their shabby clothes and bowed shoulders and unhappy eyes might pass unnoticed, or, at any rate, unrecognised.
While there is no quantitative basis that could provide evidence to his claim, the closest approximate to his hypothesis is that, perhaps, as dusk sets, those who have homes, family, and reasons to rest, would do happily take themselves to their places of origin to enjoy some form of relaxation.
Meanwhile those who are alone, restless, and wishing to go away from a demanding world, would perhaps feel more comfortable during this time of the day; the moment when it ends, and the night, slowly, begins.
The events in the story which could prove Gortsby right may be the fact that the young thief is out and about, as well as the older man. The fact that they are out there gives the reader a sense that Hyde Park at dusk is, indeed, a dangerous place to be and one where anything can basically happen. Another, possible event is that the darkness renders people more vulnerable, after all, it is the dim glow of 19th century lighting that saves people from becoming swallowed into complete darkness.
A blazing, many-tiered stretch of windows shone through the dusk and almost dispersed it, marking the haunts of those other people, who held their own in life's struggle, or at any rate had not had to admit failure.
Either way, it is all in the mind of Gortsby. He is the one assuming that there is such a time in the day where people who are dejected agree to come out. Perhaps Gortsby compares the sunlight to basking in the light of glory, while dusk is lacklustre and dark. After all, he is out in the dusk precisely because he, himself, claims to feel dejected.
He was in the mood to count himself among the defeated. ... He had failed in a more subtle ambition, and for the moment he was heart-sore and disillusioned, and not disinclined to ...labelling his fellow wanderers as they went their ways in the dark stretches between the lamp-lights.
As can be seen, the incidents in the story that state how dusk is the hour of the defeated consist on the fact that it is a vulnerable time where the darkness can be used as a time to commit petty crimes, such as the one committed on Gortsby. The rest is mostly based out of Gortsby's own assumptions and ideas.
Titling his story "Dusk" and beginning it with a description of dusk gives the setting a universal quality. Dusk is a time rather than a place. Dusk occurs everywhere all around the globe, so everyone is equally familiar with it and perhaps equally affected by it emotionally. Saki sets the story in London, but he seems to be attempting to create an impression of a worldwide phenomenon--the struggle for survival which leads to the struggle of man against man. When Gortsby thinks of so many of the passers-by as "defeated," it is a direct allusion to the theme of the struggle for existence and the survival of the fittest.
Gortsby is not a gentleman of leisure. He probably has a good job and a little extra money in his pockets. That is why he is targeted by the young rookie con man, who is one of the billions of humans who have to struggle to maintain some sort of niche in the world. Gortsby learns a lesson, which is the message or thesis of Saki's story. What Gortsby learns is that you can't trust anybody in this world and you should look out for yourself because nobody else will.
Saki never makes it clear whether the "elderly gentleman" is another con man, but there are a couple of reasons for thinking so. One is that he sat down beside Gortsby. There must have been vacant benches available at that time of evening. Another reason for thinking he might be a con man is that he seems so positive that his lost soap must be somewhere very close to where he had been sitting. It looks as if he planted it there, intending to leave and then retutn in a few minutes and use the supposedly lost soap as an excuse for starting a conversation with Gortsby. If he were to tell a hard-luck story about losing his hotel when he went out to buy the soap, that would be a coincidence but not really so strange. A good con story might be in use by any number of tricksters until it gets worn out and has to be replaced by a different one. Assuming that Gortsby was still there and that the elderly gentleman was indeed a con man, he would have the soap to substantiate his story before Gortsby even thought to ask about it. Being elderly, the second con man would have acquired more experience in using the grift through trial and error.
At any rate, dusk gives the story a universality. Dusk moves like a dark cloud as the earth turns, creating a medium in which men and women struggle to retain a place on the spinning globe by hook or by crook.