David Splane mentions babies born 10 minutes before or after because he knows that they will one day quote him and show that an overlapping generation includes a baby born 10 minutes before Freddy Franz died.
I am sure these guys didn't slow down to think of the ramifications of "life before birth."
I get what a real "overlapping" generation is. If you wanted to mention the generation of 1914, some of the people alive at 1914 would be near the end of their lives and some would be closer to the beginning, but their lives all overlap at 1914. So Watchtower is really talking about the 1992 generation with their goofy theology.