>It was the countries in Europe that colonized and exploited Africa.

That's what they meant by post-colonial.

>Russian and Chinese involvement in Africa (and the Global South in general) was a response to Western exploitation and colonization.

Russia and China don't "respond" to those things in other countries. They look for their own opportunities to take advantage and guarantee access to resources, like nearly every other country does in its own way. If nobody else was in the way, they would be doing more of it.