Remember that the book "The Decline of the West" from Oswald Spengler was published in 1918, a century ago, after WWI. By then, the US still was not considered part of the West, a denomination which was restricted to Western Europe.
Probably the data is based on PPP (Purchasing Power Parity) and not at nominal current prices because at Nominal prices, the GDP of the EU is the largest in the World ($ 17 Tr.) compared to just $ 9 Tr. in China. So at nominal prices, China´s GDP is just 50% of the EU.
Russia also could be included in the European account. And perhaps even Turkey, even if none of these countries are member states of the EU. In fact, Russia is the most populated nation in Europe. If we add European Russia and Turkey, EUROPE´s GDP is $ 20 Tr. at nominal prices.The decline of Europe is first the consequence of two consecutive World Wars and a low birthrate since the beginning of the XX Century.
However I don´t agree with the Historical data previous to the XX Century which is based on presumptions. If that data was truth, the richest and most developed Spanish colony would have been the Philipines (which were annexed in 1571 and have the name of the King of Spain Philipe II)...but the truth is tha the Philipines were the poorest and less developed of the Spanish colonies, and the Spanish Government never paid much attention to Manila because other colonies like Cuba or Peru were considered much more important.
Just ask yourself why if China was "so rich" two centuries ago, the Philipines didn´t come to be the most important Spanish colony