Very roughly speaking, DeLong’s argument is this: everyone agrees that Germany is the poster child for an advanced economy with a great manufacturing policy. And yet, their manufacturing employment has steadily declined for the past half century too, just like ours. So if this has happened to Germany, there’s not much of a case for suggesting that the US has done anything especially wrong over the past 50 years. We’ve simply evolved from a (relatively) poor manufacturing nation into a (relatively) rich services and technology nation. This has nothing much to do with trade policy, either. It’s just what rich countries do. What’s more, it’s a decidedly good thing overall, even if it does affect a smallish number of people badly.
This is not terribly different than agricultural employment. At the turn of the 20th century about half of US workers were employed in agriculture. A hundred years later as we skated past Y2K it is about 2%.