The above excerpt strikes me as a tad too dichotomous, as if a continuity of pre-colonial African history couldn't also be affected by events and policies enacted by colonial powers during their rule over the region.
“I suggest that the preoccupation of many Africans with decolonising is inseparable from our placing colonialism as the defining framework within which to understand and narrate African life and thought. Our past—designated ‘precolonial’—is understood in terms determined by colonialism, and our future—postcolonial—is tied to our obsession with leaving it behind. How we expect to do the latter while privileging colonialism in our own understanding of our history and letting it characterise our discourses beats me.... If there was never a time when any part of Africa was hermetically sealed from the rest of the global exchange of human discourses and ideas, then to suppose that what needs to be changed or made sense of in our current experience is best traced to modern European colonialism is at least implausible and possibly wrong.
His argument is that the history of Africa is one of continued interaction with the world. Africa was influenced by Christianity, Islam and other 'foreign' ideas from the start. Parts of Africa were colonised by Greeks, Romans, Arabs, Ottomans etc. not to mention other Africans who were equally 'foreign' in many cases. Moreover, Africans themselves colonised parts of Europe and the Middle East.
Why should we insist the British Gold Coast was more important than the Empire of Ghana, and isn't it a bit absurd to label African and Arab Empires as designating a purer "pre-colonial" era?
He notes that European colonial powers often supported more conservative traditional rulers and opposed modernisation, yet for many modernisation is akin to colonisation hence the nostalgic desire to return to a mythical pre-colonial past.
Since people are largely shaped by the educational, religious, geographical, socioeconomic, and political circumstances of their societies—all of which are tangible, material conditions that colonialism can affect and has affected—I think it is impossible to accurately put a society's actions into perspective without analyzing this backdrop. There's no magical cut-off point after which the primary and secondary effects of colonialism on a society completely disappear, especially when they bleed into other aspects that in turn have their own effects, such as the proliferation of anti-contraceptive beliefs in some countries following Christian missionary work whose effects have lingered far past the official "end date" of the peak of missionary work in Africa.
That's largely his point.
There is no cut off point and everything bleeds into everything else and thus no reason to grant European colonialism exceptional status with regard to earlier Arab, African, etc. colonialism.
For something like Christianity, it has been in Africa longer than most of 'Europe'. The 20th C expansion of Christianity was largely driven by African missionaries and evangelists, not Europeans. It is almost comical how inept European missionaries were when you read their histories, often they would get literally no converts in a decade (before dying of malaria).
The seedbed of missionary schools etc would have some long term impact of course, but the Africanisation of Christianity by Africans is responsible for the massive 20th C growth.
Also the idea that 'European' Catholicism is significantly responsible for lack of contraceptive use is very dubious given religious demographics
This is not to say that Africans have no agency or that human nature won't consistently lead to the same results no matter where one looks in the world, and Africa is no exception. It's just that the way people exercise their agency is still informed and influenced by numerous factors that are inevitably shaped whether partially or fully by past and current circumstances. It's no coincidence that many former French and British colonies still speak French and English respectively, for example, despite having agency to change their official languages to completely exclude both (which would be impractical and extremely unrealistic, despite being theoretically possible).
We can say the same about why so many speak Bantu languages or Arabic. As someone said "we are here because the past happened".
It's like in Europe where people want to believe the Enlightenment was a break from the past rather than its latest iteration. Periodisation of history tends to create magically divisions between eras, rather than convenient literary constructs to note when certain long term trends become apparent. It's like believing it was only the last straw that broke the camel's back.
The designation "African" is a very arbitrary construct, but for many (not saying you) white colonisation of brown/black people is somehow different from black/brown colonisation of black of black/brown people or white colonisation of white.
(In parts of South Africa, white colonisers have been there longer than Zulu colonisers. Both used violence to displace the 'traditional owners of the land' yet only the former are considered colonisers).
His argument is rejecting the framing allows people to start to appreciate the complexity of history, rather than the facile "oppressor/oppressed" version harking back to a mythical past that is favoured by "decolonisers".