I think the definition of "middle" class is based on the local standard of living.
No it is not about home ownership or cars or how many toys you own, but IMO it's about whether you have enough income to pay for more than just food and shelter, and afford education for your kids plus some day to day expense that you can afford to pay for (i.e health care, entertainment once in a while, savings is trending up and you do not have to borrow money month to month to keep the lights on, etc).
Nice article / slides though, this shows why energy cost would keep going up despite all the efficiency improvement: there are a lot more middle class people now than before.