From the article: "Google Now has a huge knowledge graph—you can ask questions like ‘Where was Abraham Lincoln born?’ And it can name the city. You can also say, ‘What is the population?’ of a city and it’ll bring up a chart and answer. But you cannot say, ‘What is the population of the city where Abraham Lincoln was born?’”

Out of interest I tried this question on WolframAlpha and it happily returned the answer[0].

[0] http://www.wolframalpha.com/input/?i=What%20is%20the%20popul...