The author takes PPP (Purchase Power Parity) per capita of the UK as a whole versus individual U.S. states. So what does it mean that Mississippi on average has a higher PPP than the UK? Two things:

The UK gets dragged down by it’s poorer regions; and

The U.S. has enough ultra-rich people to drag its PPP up despite a large swathe of its population being poor.

Another way to look at it is, if wealth distribution was fair in the U.S., even people in Mississippi would be better off than the average Brit.

You should read the last paragraph of the article.

I did, and it’s a vast argument to make based on two data points. That the bottom 10% of the U.S. and Finland have the same PPP says nothing about the wealth of the rest of the population, just that the rock bottom of both countries is more or less the same. Not to mention, socialized services probably means Finland and Sweden’s bottom 10% probably has higher life expectancy than the U.S.'s bottom 10%.