Diamond on Japan's History and Uniqueness

Hat tip Megan McArdle.  A couple sentences:

. Unlike the winter rains prevailing over much of Europe, Japan's rains are concentrated in the summer growing season, giving it the highest plant productivity of any nation in the temperate zones. While 80 percent of Japan's land consists of mountains unsuitable for agriculture and only 14 percent is farmland, an average square mile of that farmland is so fertile that it supports eight times as many people as does an average square mile of British farmland. Japan's high rainfall also ensures a quickly regenerated forest after logging. Despite thousands of years of dense human occupation, Japan still offers visitors a first impression of greenness because 70 percent of its land is still covered by forest.

This is an old Jared Diamond piece on Japanese prehistory. What does the factoid mean in the context of concerns over modern agriculture?