Yep, basically he points out that we currently have 4 generations alive at once as we're living much longer. Each generation is multiplying and branching out, so it's a double wham effect.
If 2 adults have 2 kids who produce 2 kids each, you've got 8 people alive within 3 generations, but the grandparents are still kicking on too.
Exponential growth, which make it easy to make a fairly accurate mathematical prediction.

