Many unstable countries can directly tie their current tribal problems (a major source of war and strife in Africa) back to colonial policies. It's not an excuse, but it is definitely partially to blame.
Let's take the DRC since the article is about them. When King Leopold claimed the Congo Free State, he introduced a regime where atrocities were common-place. It was infamous even by colonial standards. When it became the Belgian Congo (1908), the worst crimes stopped, but racism/segregation and forced labor were still commonplace. After independence (1960), they elected president Lumumba. Looking for international help to stabilize the new country, Lumumba was turned away by the US, but got an agreement from the USSR to provide aid. Upon hearing this, the US set up an embargo to prevent Congo from receiving any of the arms and then supported a coup by Mobuto which led to Lumumba's murder. Mobuto, supported by the anti-USSR powers, made himself fabulously rich and ran the economy and government of the DRC (one of the most resource rich countries on earth) into the gutter. In 1997 Kabila's rebellion pushed out Mobuto and there has been war and rebellions for the better part of the past 22 years. Given its rich resources and poor government control, other nearby African nations have encouraged the conflict so that they are able to profit from the resources (most notably minerals into Uganda and Rwanda). The DRC never even had a chance to establish a good government
Everyone is responsible for their actions and history isn't an excuse, but it's pretty easy to see how a history of resource exploitation and human rights abuse encouraged by western powers has led directly to a country full of resource exploitation and human rights abuse. African colonies are not ancient history.