Rather than comparing social science to physics, chemistry, biology, and engineering, a more useful comparison might be to history. Historians know lots, both about specific things like what products were made by people in city X in century Y, or who signed treaty Z, and also about bigger trends in national and world events. But historians haven't given us any useful products. History has value in itself--interesting stories--and helps us understand our world, although not always in a direct way. Once people start trying to organize their historical knowledge, this leads into political science.
Obviously I think this is true now--it is, essentially, one of the thirty-nine articles of faith I accepted when I became a social scientist!--but there was a time when I did not believe this, and I am aware that there are many people who believe that historical knowledge cannot be organized: what was, was, and what will be, will be, but the only connection between them is sequential, not comparative. That is, whatever causal arrows exist are monodirectional and hence noncomparable (except, I suppose, when you discover the Roman planet*). Something similar exists whenever hardcore area specialists from different areas meet: they may recognize similar structures in their regions, but they will either believe that their regions are essentially different or lack the vocabulary to make useful comparisons.
Gelman's conception of history, then, is not one that any historian would recognize, but then I doubt that the conception of "History" that most people carry in their heads is any different from Gelman's. If ordinary people had a historians' understanding of the past, they wouldn't look for "lessons of history"--which may mean that the demotic understanding of history is actually a naive version of social science.
*"Slaves ... and gladiators. What are we seeing? Twentieth-century Rome?" And, amazingly, twentieth-century Rome looks a lot like twentieth-century southern California. ("The word was 'smog'.")
Postscript: On the other hand, when Tom Lehrer pronounces you anathema ...