I've read her book and while it's possible, I don't find it convincing that many Somalis were ever Christians. There may have been some, or perhaps some Xabeshas in the hinterlands of Woqooyi who practiced it (though this is dubious too when I really look at it) but it doesn't entirely jibe with the linguistics, for example.
The Somali language, both north and south, is teeming with evidence that Somalis practiced Waaqism. Just a sheer abundance of words and surviving folktales and it shows in the material culture and customs both north and south. There really isn't as much proof at all in terms of Christianity. Just some tribes doing pilgrimages to some old saint sites and marking crosses on their foreheads and stuff like what Sada Mire found archaeologically which isn't THAT extensive, to be honest. I think it's a stretch to believe the whole northwest was Christian. But who knows.