Earlier quoted context omitted.
This comes across a little patronizing. The poster is making the point that North Africa had been part of a civilized empire for centuries, and full of wealthy cities, some famous for their learning. Some of history’s greatest minds lived in Roman North Africa. These cities were conquered. Some were sacked and destroyed. There was major loss of life, economic destruction, and loss of property, including written works…
Much of Roman Africa was in a state of deterioration due to Christan invaders like Visigoths(I think) who overrun the Roman African provinces towards the decline of the Western Roman Empire. At the time of the expansion of Islam, there was no longer even a Western Roman Empire.
Later in one of the most dashing military campaigns, the Eastern Roman Empire(Byzantines) forces under Belisarius captured most of the territory of the former Western Roman Empire, including the African provinces after defeating Vandals. The African provinces were then organized into the Exarchate, which is what Muslim armies eventually overcame in the 690s AD.