Digging into a couple of meaningful topics within one discipline teaches someone the practice of that discipline and trains the mind to think about context, relationships, and connections. He's talking about learning to think, then applying knowledge and skills to new problems and topics within that discipline, with the possible spinoff of cross-discipline applications as well. This approach takes away the pressure to "learn" thousands of facts:
"The notion of coverage, of going through a bunch of disciplines, and learning facts and concepts, is assessed by schools all over the world. It's never been a very good idea, but now it's really irrelevant. I would throw away 95 percent of the coverage that we do; figure out really important questions and issues, and give people lots and lots of time to learn about how disciplined minds think about those issues, and then to practice those disciplines themselves."Right now, an average high school student studies might take two history courses between Grade 9 and 12. One might cover all the important points from 200 years of national history in five months. Maybe the other one tries to pull out the highlights of 500 years of European history with the same amount of class time. Even if those courses are exceptionally well-taught, the odds of the student retaining much of that information past high school is close to zero, and more importantly, they aren't learning anything about the process of thinking about historical problems.
Now, discard that traditional view of high school history. What if history students could spend a whole year studying the holocaust? Think of the web of knowledge you could build around that concept, stretching back into the roots of racism, digging into the geopolitical climate of the years following World War 1, analyzing the economic and social costs of World War 2, studying the personal stories, images and art of the people involved, and extending it into the creation of the Israeli state and the United Nations.
Ideally, the students could find areas of interest within those parameters, even choosing to focus on a relatively small number of sub-topics depending on what stories were compelling to them. But most importantly, they would learn how to find the resources required to really understand some aspect of what happened in the past. Each student could then use the processes and skills they've learned (and perhaps some of the historical knowledge as well) to learn independently about any other period in history. Once you have the cognitive tools, finding the information is easy, especially in the information age.