Did Marx get something right?

I'm not sure how this passes as insight at theEconomist's blog but here it is:

The Prebisch-Singer hypothesis (PSH) was a staple of leftist economics during the second half of the 20th century. Raúl Prebisch and Hans Singer, working independently, showed that the “terms of trade” between primary products and manufactured goods tended to decline over time. In other words, producers of crops and raw materials gradually became poorer relative to producers of cars and household appliances. If true, the theory would have important implications for world trade; it would suggest that commodity-focused economies must diversify into other sectors or risk falling ever further behind richer countries.

The evidence they show is how commodity prices have gone down over time - which to me seems to fit more with Julian Simon's views (WSJ) than really any validation of Marx's other views.

So countries that create value using commodities get wealthier than countries that just produce commodities. Hard not to be sarcastic here, but put me in the camp that's not shocked.