This is an interesting question. It is a very strange coincidence that both the old man and the young man claim to have lost a cake of soap. It is hard to see how they could have been accomplices, but it seems likely that they are both grifters working the same grift. This is not unusual. When some grifter invents a good story, it is picked up by other grifters and used on the public until it becomes ineffective through overuse. Assuming the old man is a con artist, he must have been planning to leave the cake of soap near the bench and then come back to pretend to search for it, using it as an excuse to start a conversation with Gortsby. But in the meantime Gortsby has found the soap and run off to give it to the surprised young man. The old man would be the better con artist because he had the foresight, acquired from experience, to buy a cake of soap ahead of time in order to substantiate his story. But it could just be a strange coincidence that the old man had really dropped a cake of soap by accident and had no intention of trying to swindle Gortsby. It is hard to guess Saki's intention with this surprise ending. I tend to guess that he wanted the reader to believe that the old man was another con artist who would have told Gortsby substantially the same story that the young man told about having lost his hotel and needing to "borrow" enough to rent a room for the night at a different hotel. Such an ending would be in keeping with Saki's misanthropic philosophy and Tory politics by suggesting that society is full of people who want to get by without working.
If the "elderly gentleman" really wanted to use the same scam on Gortsby, why wouldn't he just strike up a conversation the same way that the young stranger did when he took his place on the bench? It may be that a young man could strike up a conversation with another young man much more easily than an elderly gentleman could do. If the old man is really a grifter, his approach seems more ingenious than that of the young man, who may be completely new at the game and still learning by experience.