Notice it says "one of". Nothing you have posted makes what he has written untrue.
So you are telling me you can't see the difference between the economy of Poland and the other countries? You know what? Let me search a little longer and will come up with similar stories about the economies of other European countries. And you will see this wonderful Polish economy of yours slowly dropping down the list.
Some more quotes from
http://en.wikipedia.org/wiki/Polish%E2%80%93Lithuanian_Commonwealth#Economy:- The economic strength of Commonwealth grain trade waned from the late 17th century on.
- The Commonwealth's major attempts at countering this problem and improving productivity consisted of increasing serfs' workload and further restricting their freedoms in a process known as second serfdom
- The Commonwealth's preoccupation with agriculture, coupled with the szlachta's privileged position when compared to the bourgeoisie, resulted in a fairly slow process of urbanization and thus a rather slow development of industries.
- From Gdańsk, ships, mostly from the Netherlands and Flanders, carried the grain to ports such as Antwerp and Amsterdam.[63][64] Besides grain, other seaborne exports included carminic acid from Polish cochineal, lumber and wood-related products such as ash, and tar.[58] The land routes, mostly to the German lands of the Holy Roman Empire such as the cities of Leipzig and Nuremberg, were used for export of live cattle (herds of around 50,000 head) hides, furs, salt, tobacco, hemp, cotton (mostly from Greater Poland) and linen
- Somewhere between the 16th and 17th centuries, the Commonwealth's trade balance shifted from positive to negative.