Interesting article but there must be massive multiple variables.
You've probably heard it at some point: A friend claims his older vehicle is far greener than the new car with much higher fuel economy you just bought That's because, he says, the old car didn't require all the energy and natural resources your car consumed as it was built. But is that older...
www.greencarreports.com
A massive exec vehicle such as a QP must take much more to build and the MPG is much lower so the variables are lower. Why they worked on 15k miles per year I don't know as that isn't the average.
There are also so many hidden variables that have likely not been calculated. Less or no new cars would need so much advertising, marketing and all sorts. How many adverts, brochures and lots of things you wouldn't think of or calculate would no longer be needed. All have an impact.
An accurate study is difficult but not sure they have this one fully completed correctly.
Mileage completed obviously makes a massive difference.