They were decedents of Roman people living there and not sub Saharan Africans.
The Roman people living there being ethically Berber, that is, African. The name Africa itself probably coming from the Afri Berber tribe. They were both Roman people and also ethnically African.
And Berber tribes despite not being from subsaharan Africa did, through thousands of years of contact, have significant ethnic and cultural admixture with various subsaharan African peoples. Not that North Africa is less African than subsaharan Africa.
https://en.wikipedia.org/wiki/Macrinus
> Macrinus was born in Caesarea (modern Cherchell, Algeria) in the Roman province of Mauretania Caesariensis to an equestrian family of Berber origins. According to David Potter, his family traced its origins to the Berber tribes of the region and his pierced ear was an indication of his Berber heritage.
Many of the top people in large empires aren't the same race or culture of the original people.
The caliphates had many groups not from the ruling elite's ethnic groups gaining high positions.
The romans had warriors and leaders of many different origins.
The mongol empire had and frequently used different ethnic groups in their armies and their leaders race-mixed with the locals.
Empires tend to be more colorful in order to control more land, its a natural effect of owning a lot of land.
https://www.heritagedaily.com/2016/05/ancient-dna-study-find...
North Africans are and always were the same people genetically. The North African emperors I am referring to specifically had Berber, not Punic or Phoenician ancestry. Which is still the dominant ancestry in North Africa today, and the Berber culture and language still lives on today.
Unfortunately it seems you are not knowledgeable of the history either of late Rome, of Carthage or of (North) Africa in general, and the authors of that website either did an incredibly poor job at it or are as well.
The same goes for early europe. Plenty of people from either. There were Roman emperors who were indigenous to (Northern) Africa.