To some degree, man, I don't fully trust this Burton. They had weird tendencies but borderline non-believers are a stretch.
There is also a difference between not being learned and being a non-believer. They would probably have lost their shit if someone claiming to be their kin said they rejected there was a God, instead of not praying 5 times a day. Latter does not mean the former. Binding in that sense likely means they don't do their Islamic duties. Because in other texts you will clearly read that Somalis were very much believers, lacked the fear of death, and had a very intrinsic belief in the wisdom that people die and that God watches over them. The fearlessness reflected their trust in their deen, although they might not have been learned in the deen.