Those Somalis got it from the Samburu. The rendile apostated and adopted some of their cultural practises which is not known to most
rendille might even be of garre origin. Here is what I found
"Tradition says that before the Sakuye attached themselves to the Borana, they lived with the Rendille in the Mt. Marsabit area. The Sakuye and the Rendille are said to have broken away in the 1500's from the early Somali group from which the Garre also developed. Tradition also says that the Gabbra Miigo, the Sakuye Miigo and the Gabbra of Kenya's Eastern Province later originated directly from the Garre Somali."
well now we know it's garre who practice the blood drinking and not degodia. The former seems to have many connections to borana and other small oromo groups in kenya.