Africa was hardly an unchanged society in the 1870s, by then contact with the Europeans (and I don't just mean the Slave Trade here) had changed the nature of the continent drastically. There is no reason to think that had Europe 'left it alone'*, it would at all like it did in 1870.
* (Of course, what do you mean by that?)
Who said anything about "it would look like 1870"? It obviously would not. The interesting thing is how the continent would have developed without conquest and colonization. No one is making a case for total isolation, that would be ludicrous.
Left it alone= not colonized it
The point was partly to pre-empt the common notions of pre-colonial Africa. Of course by 1870 there was British Cape Colony (whose white population was of majority Dutch descent and had moved much further into interior), French Algeria and all those coastal forts that had been left behind from the days of the slave trade which nobody was quite sure what to do with. Also strong (and by then a few centuries old in places) Portuguese presence in certain parts. Though at this place the Congo "Free State" was still a notion in a megalo-and-mono-maniacs perverse ramblings, taken seriously by none.
But what "not colonize" really mean in this context?
Coastal forts would probably be abandoned, as they no longer had much relevance after the end of the slave trade.
Portuguese settlements are either not expanded to the interior
or are abandoned, as their establishment and relevance were strongly related to the slave trade (Portugal being slave trader no. 1).
South Africa and French Algeria with their significant white settler population are a different story all together. They are also on the fringe of the continent.
The question relates to the conquest
of and establishment of colonial administrations
on almost the entire continent 1880-1900.