Yes, their birth rate assumptions would only yield the population milestones they suggest if the life expectancy was significantly below the reported level.
We use the world population and average life expectancy to calculate births in a year. To connect these point estimates at the varying times in history we use a bounded exponential growth model for births.
Last night an Egyptian in Toronto, who felt powerless to help in the midst of the bloodshed, asked if we could create a place for Egyptians and the world to remember the lives that have been lost in the protests in Egypt.