First of all he reiterates the mantra that GM has been repeating all year of energy diversity. In the future there are likely to be many different fuels and energy sources in use for transportation. He goes on to explain that the size of battery that would be needed for a useful series hybrid would take to long to charge and be too large and heavy to make it perform better than a parallel hybrid. He also mentions that said batteries haven't even been fully developed yet.
Mr. Miller and Toyota may well turn out to be correct in their assessments of series hybrids in the long run. However, there was plenty of skepticism about the work that Toyota did on parallel hybrids over a decade ago and if they hadn't proceeded anyway we probably wouldn't be where we are today. No one has abandoned parallel hybrid development in favor of series. They and other technologies are continuing on parallel paths. Ultimately we may well end up using all or none of these. But for now let's see where they go. Thanks to Irv Miller for chiming in.