Is it true Islam spread throughout Africa via Arabs?
That is incorrect, Islam spread throughout Africa in this typical way. Arab merchants, Scholars and Sufi Mystics came into contact with the African elite of society. These elites converted to Islam. From this a large part of the population accepted Islam. From this large powerful African Muslim empires (Mali Empire, Songhai Empire, Sokoto Caliphate) grew and they expanded there empires in turn influence of Islam grew. These where Africans expanding there empires doing the same thing that there counterpart Africans that practiced indigenous African religions did, the only difference is they where Muslims.
When it comes to African countries that have a Majority Arab population like Egypt and Libya. Arabs conquered these nations from the Roman or Byzantine empires. Arabs conquered Libya in 647 from the Romans. The Romans annexed Libya in 74 B.C. So the indigenous people of these regions where defeated and conquered by the Romans and converted to Christianity, why before the Arabs arrived
There is more Muslims in Sub-Sahara Africa then North Africa and the entire middle east combined. You cannot equate Islam in Africa with Arab colonization and Christianity with European colonization.
There is no country in West Africa that has a large Muslim population that speaks Arabic practically or officially. But every country that has a large Christian population speak either French, English or any other European language as there official language.
The list below shows the African Countries with significant Muslim population and how Islam introduced and spread
Country- Reason for spread of Islam
Benin - Hausa, and Songhai-Dendi Merchants
Burkina Faso - Mossi Kingdoms
Cameroon - Fulani, a pastoral nomadic group through commercial activity and Sufi brotherhoods (African)
Chad - Contact with West African Muslim Merchants and pilgrims
Ivory Cost - Muslim Berber Merchants & Ghana empire
Djibouti - Adal Sultanate
Egypt - Muslim Arab empire defeated the Byzatine empire
Eritrea - Nomadic Tigre people
Ethopia - Muslims been ariving in Ethopia since, Muhhamad sent the first pilgrims
Gambia - through Sufi brotherhoods
Guinea - Wassoulou Empire (African)
Guinea-Bissau - Recent conversions and Mali Empire
Libya - Muslim Arab conquer Roman Provence of Cyrenaica
Mali - Berber and Tuareg merchants, Sufi brotherhoods, Mali Empire
Mauritania - Muslim Merchants and craftsmen, Sufi brotherhoods
Morocco - Muslim Arab conquer Roman Provence
Niger - Songhai Empire
Nigeria - Bornu Empire, Mali Empire
Senegal - Takrur Kingdom
Sierra Leone - Mande tribesmen
Sudan - Muslim Sultanate defeted Nubian Christian kingdoms