There are 7 billion people on the planet and the article cherry picks the poorest 3.5 billion. I imagine that there is little opportunity at that level. I doubt if anyone in the United States is on that list.
Also, many middle class people, such as myself, have negative total assets due to debt. Because 'wealth' can refer to not only money but general capital (including human capital, which is what I have invested in) I would not say that my total wealth is less than zero, and I definitely have a lot of opportunity, privilege, and comfort. But if I was writing an article for the Guardian in order to rile people up about inequality, I think I could not incorrectly state that I have negative wealth, and by net $$ standards far less money than many of the world's destitute.
Source?
The actual data on this appears likely to refute your claim:
Yes, that’s right: America’s poorest are, as a group, about as rich as India’s richest. [1]
That's at least the better part of a billion right there America's poor are wealthier than in India. According to the graph in the article, America's poorest are also wealthier than nearly all of China (1.3 billion officially, 1.5 billion unofficially), and 2/3 of Brazil. Not including the rest of Asia, Africa, Latin America, and Eastern Europe.
[1] http://economix.blogs.nytimes.com/2011/01/31/the-haves-and-t...
And I'm not sure what to say about your negative wealth debate. First world problems?
They probably wouldn't have been billionaires, but they would have had a good chance at becoming middle-class in Brazil.