Europeans have been working trade deals with Africans for generations. But the deals have been exploitative of accessing mineral, ore, oil, agricultural, fishery, and other natural resources. This is likely to remain the case, since Africans have never shown any ability to deliver added value needed for modern industrial development. Its fine for the UK, though, if they can make lucrative deals for the natural resources they can apply elsewhere to their factories in the developed world. Brexit had no role to play in any of this. Why the writer makes this connection is not clear.
It has a role in the sense that, without it, the UK was unable to negotiate trade deals on its own behalf.