So, the top 10% of the US is the top 1% of the world. So, great, I'm still in the 99%, even if all 7 billion are counted.
The true global middle class, falls far short of owning a home, having a car in a driveway, saving for retirement and sending their kids to college. In fact, people at the world's true middle -- as defined by median income -- live on just $1,225 a year. (And, yes, Milanovic's numbers are adjusted to account for different costs of living across the globe.)
I would also like to point out that this is impossible to compare to; and also that the majority of the U.S., that houses these world's richest, can't afford to own a home, save for retirement, or send their kids to college (a car is a necessity in most of the country, so whether it's in a driveway or parking garage is not all that important).
A subsistence farmer in South America, FI, selling excess, could be in better shape than a typical person in the U.S. making 20k+/yr (assuming no welfare). A person in one nation may not have reasonable options to lower their costs of living to the same levels as somewhere else. As long as that is the case, any normalized dollar to dollar comparison is propaganda.
While still highly flawed, a decent comparison, at least, would be comparing consumed commodities, counting using your own production the same as pure consumption (if I buy food, that's part of COL; but if you make your food, only external inputs are COL; that's skewed from the start, and only gets worse as technology is factored in). Then, count in paying off any debts as a negative against current wages. Then taking what's left over after that type of COL comparison, and compare it to local commodity and service costs (my $20 won't get me as much as a Panamanian's $20). Then, the ratio of the two would be the comparative advantage. Developed nations would still be on top, and there would still be holes, but it would remove the degree of absurdity that these types of studies exhibit, and at least be a step towards being able to measure meaningful wealth by income and outgo.
I have suspicious feeling, though, that any such work would have a hard time finding proper funding.