I would say Africa isn't the biggest. Vietnam (mentioned in article) and Bangladesh already produce a lot and are overtaking certain industries like textiles and shoes. For example, Nike's biggest amount of contract factories are Vietnamese and they earn roughly one to two dollars per day per Vietnamese worker.

For now, Western companies are still choosing countries in Asia. Maybe Chinese companies are choosing African countries because minimum wage has increased in China and many factories have gone bankrupt and other Asian countries only cater to Western countries for their factory needs.