Ohh, I think I know where the author got the vibe that Nairobi is "violent and slum filled" - and the images too

There is a Discovery Channel dude who goes around the globe documenting the 'organised gangs' of the world...

The guy came to Dandora, Mathare and other slums  to hunt for Mungiki and was really shocked to see the trash, the black nairobi river was filmed several times [probably why government cleaned it] -

The Discovery Channel dude was so touched by the strife there that he came back for another documentary,,,,  He also set up a foundation to help a few kids.


Ref: Discovery Channel : Gang Nation - by Ross Kemp - A Kenya special
                                    And a 20 min follow up  - The Glue Kids