I get that, but how exactly do Joseph and his brothers being called a "generation" even relate to generations overlapping? I'm just trying to see where their reasoning comes from.
In the sense that they were all from a specific time frame. Joseph was what the 10th, or 11th of them I believe? He wasn't the youngest, nor the eldest. They used Exodus 1:6, "eventually Joseph died, and also all his brothers and all that generation." What the GB is attempting to get across is that "this generation" isn't indefinate. It has an ending. In the text from Exodus, it mentions Joseph and his brothers and those from their time, I'm guessing within the same age demographic as the 12 of them. The setting for the next generation of Israelites was after those from the era of Joseph and his brothers had passed the scene. But there were 12 brothers, all of different ages. The same as the annointed who were on hand to recognize the significance of the events in 1914 onward. Also, those who may have been too young to recognize 1914's importance, or not even born, but nonetheless who had the heavenly calling while those who originally recognized the events of 1914 were still alive.
They specifically used Fred Franz as one of the original annointed who recoginized the importance of 1914. Those with the heavenly calling after him, who although younger than Franz, and weren't around nor recognized 1914 as being important, where still contemporaries of Fred Franz being alive with him in the same era.