This novel definitely shows the truth of the statement through its setting at the time of Kenya's struggle for independence and the way that it focuses on the lives of characters before and after who are involved in the independence struggle and the kind of challenges and trials they face.
Let us remember that this classic novel by the author actually is set in the years prior to independence and features the actual moment of Kenyan independence itself, which occurred on 12th December, 1963. Clearly, the context of Kenyan independence is at the heart of this novel, as so many characters work towards it and have their actions motivated by this aim.
However, at the same time, we can argue that the author uses this context to provide a compelling "co-text," or a text that comments upon the central context. The way in which he focuses on the drama surrounding the betrayal of Kihika and the feeling that some characters, namely Warui and Wambui, have that maybe things haven't changed at all through the gaining of independence, act as a critical commentary on this central historical event.
I think the prime theme of this quote is that history cannot just be understood in isolation. It also involves an understanding of the way in which there are many different "histories" of any one event, and in making our judgement and drawing conclusions about any event, we need to take into account not just the context or the surrounding details of that event, but the many different "texts" that are produced by the various people involved in that event.
This novel does that through focusing on the ways in which a group of characters each have their own "text" or their own story to tell in relation to Kenya's colonial relationship with the United Kingdom and the story of the emerging nation as it slowly develops and gains independence. What is so interesting about so many of Ngugi's works is the way that he selects a group of about five or six characters to reflect the main ideas and responses to the problems that Kenya as a nation who is struggling with its independence faces. Ngugi therefore both examines the context of his country's history, but then also pays equal attention to the "co-text" through an analysis of the different version of the same historical events, as seen and viewed through the eyes of very different characters.