According to a paper on the Eurasia Review, 'US trade policies inadvertently turned Africa into a temporary trade corridor for China.'
In summary, they authors say that AGOA helped to achieve its aim of spurring the growth of U.S.-exporting African enterprises, especially clothes, from its inception to about 2004. From 2005, Chinese apparel exports to the U.S. no longer faced quotas as before, making it more viable to manufacture and export clothes from China to the U.S.
They say AGOA's benefits to African clothes-exporting countries under AGOA diminished from then on, because they had actually been driven by Chinese manufactures coming to AGOA-eligible countries to bypass the U.S. quotas on Chinese clothes.
The Rise And Fall Of (Chinese) African Apparel Exports is required reading.