From the article:
"Drawing more people into cities could help significantly shrink the country’s overall greenhouse gas emissions. Low-density developments produced nearly four times the greenhouse gas emissions of high-density alternatives, with research finding that doubling urban density can reduce carbon pollution from household travel by nearly half and residential energy use by more than a third."
This refers to the overall volume of pollution, but not the concentration of pollution in a particular location. If you're from the UK you've likely heard about Ella Adoo-Kissi-Debrah, who tragically died aged nine in 2013 after an asthma attack. Last year a coroner's verdict was given stating pollution played a part in her death, specifically the high levels of nitrogen dioxide. I know this is just one case, but it makes me think the fine structure of urban planning is very important in this respect.
I'm aware of the philosophy behind HN and the dictionary entry alludes to this. From my experience it's more focussed on technology and a liking of knowledge in general.