Sorry but you are completely incorrect on the concept of foreign. Look at what the basic definition of foreign isIt doesn't matter how long it's been in Africa, if it came from somewhere else and didn't originate on the continent it is still foreign.
Foreign: strange and unfamiliar.
Foreign does not have anything to do with originality. Something cannot be foreign to a society, once it's been established there for a significant period of time. So using your logic, English is a foreign language to England, since the Germanic invaders introduced English to England and replaced the previous original Celtic language during the 9th century, 200 years after Islam was introduced to Africa. So Islam was established in Africa, before England was speaking English. When a foreign disease is introduced into a new society the people get sick until they build immunity to it. Then guess what its no longer foreign.