What was special was not the geography, but the war. Guthrie is referring to the early experience that many commanders who took part in the Thirty Years War gained by fighting in the Dutch Revolt – which began in 1568 and went on (with significant truces) until 1648.

The war was largely fought between Spanish and Dutch forces, though both recruited mercenaries, and it was not only one of the longest-lasting but also one of the most aggressively-fought series of campaigns that took place anywhere in Europe in this period - making it an effective training ground - though it was the war that offered the training, rather than their being some specific military training school in the province. There was also a connection between the Spanish Netherlands and the Holy Roman Empire, in that Habsburgs ruled over both territories.

Tilly was actually born in Brabant, part of the Spanish Netherlands, which makes his participation in the war against the Dutch on the Spanish side not especially remarkable.

Source

Olaf Asbach & Peter Schröder, The Ashgate Research Companion to the Thirty Years' War