By now I have read at least three or four different theories of how the Rendille link in to the Somali clan system - all from a Somali perspective. (My French is almost non-existent so I haven't read the book you recommended, but it also seems to come from a Somali perspective.) But then, I've seen Somali claims to almost any tribe in the Kenya, including the Masai and Samburu!
My view on the issue was changed radically when I read GΓΌnther Schlee's book "Identities on the Move: Pastoralism and Clanship in Northern Kenya". He comes very much from a Rendille perspective as he has done his PhD on the Rendille people.
His argument is that the Rendille, Gabbra, Sakuye and Garre (and maybe even the Ajuran of Kenya) were all one people group about 500 years ago. They then split and became infuenced by different neighboring groups: The Rendille by the Samburu, the Gabbra and Sakuye by the Borana and the Garre by the Somali.