>For example a dairy farmer in Switzerland earns more than one in Poland because people pay more for milk products in Switzerland compared to Poland.
A Swiss farmer is much more productive when measured in currency units, but probably not that much in milk volume.
However as we are talking about salary differences it's money that matters.
>So why would the GDP per capita of their country be a useful tool in deciding who to hire?
Why would it be? In this conceptual model the ability of a developer is how much he multiplies the output of whatever he's working on, but his productivity is the absolute value of added output. How could it be counted otherwise, in what? Lines of code?
If productivity didn't depend on location immigration wouldn't exist.