He is part of a group revisionist southern tribalist historians, who creates this imagined Southern vs Northern dichotomy. And has been called by other historians and writers claim them to be heightened with regional ethnocentrism.
So he tries to shun the north a lot and create imagined history for the south to posit against them, his basis for the South being islamized earlier is the fradulent Al-Zunuj manuscript, thats been written off by modern historians. And some vague Arab reference to navigational difficulties in the north.
No other sources collaborates those claims and the earliest sources that mention the south talk about them as mostly pagan Al-Idrisi and Al-Masudi , becoming fully islamized by 1200s
Whereas Zayla is mentioned at the earliest 800s to have Muslims , and being an Islamic state in the late 900s and archeology pretty much shows the inhabitants in the interior were Muslim around 700s eating a halal diet.
Excavations by archaeologists in Ethiopia have discovered that early Muslim communities ate a cosmopolitan diet as the region became a trading centre for luxury goods. - HeritageDaily - Archaeology News
www.heritagedaily.com
The Eastern coast was also mentioned to have converted and had an organized sultanate with coastal towns around early mid 900s
Also there has been shown that not only was there chronology of towns in the North , specifically the North Western region was cultivated with agro-pastoral farmers much like the south was and stretched into the Hararghe/Showa/Awash area.
Even people between Zayla and Berbera people farmed, according to medieval description
Both the South and the North had a significant Semi-nomadic pastoralists population alongside them.