Lots of people are making the entirely reasonable and correct point that slums are not very nice places to live. Nevertheless, the article is correct in saying that slums engender efficient, low-impact living, when compared to the lifestyles of the rich (anyone reading this is almost certainly rich, in global terms).
I've heard various people say things along the lines of "you want to see an environmental disaster area - go and look at a slum". This thinking attempts to sidestep the responsibility of the rich by blaming the poor, who are in fact relatively guiltless. We might not want to live there, but we should jettison the popular idea that slums are bad ecologically, as it is the reverse of the truth.