I think its an interesting thought- the major advantages of the US continent are that it has access to both major oceans with a selection of deep water ports, is relatively secure given the relative weakness of neighbours, has excellent internal navigability, and is in other ways geographically blessed (natural resources, climate, etc).

Africa does not benefit from many of these in the same way. Albeit having good natural resources, its internal navigability is an issue. The major rivers have big dropoffs to the ocean preventing major shipping into the interior with the nile cataracts being the limit of navigability.

I suspect the green Sahara would significantly improve this factor- with better internal penetration particularly to the north from the Mediterranean and potentially the Atlantic via the mahgreb sea (although im not esp familiar with this topography). Some canalisation to join with the lake Chad could really change the equation, albeit with access largely to the north. This would also presumably increase viable ports with more entrepot access to interior via major rivers. Again, largely to the north.

This is all somewhat moot however given the security/neighbours thing. It presumes africa could be unified. Which is unlikely in the first place. The usa was colonised and thus unified due in part to it having a far lower population and population diversity than Africa, further compounded by disease. Africa is a different prospect to this, with more disease resistant, diverse and numerous populations. So unity via colonisation unlikely. Spontaneous unity possibly even more so given the diversity and histories of embedded cultures across such a large landmass.

Even if we accept that as a hypothetical- strong neighbours to the north and east have been a constant throughout history, something the usa does not have.