Ibn Tumart came from the Hargha, a Masmuda group in the Sous on the Atlantic side of the Anti-Atlas, and the movement he built was made out of the Masmuda tribes of the High Atlas. That matters for how his teaching is described. He preached in Tamazight, his creed was put into Tamazight for his followers, and the call to prayer and the basic instruction of the movement were given in it. The surviving Book of Ibn Tumart is in Arabic, but it records a teaching that reached most of its audience in another language.
His theology was a hard reading of tawhid, the unity of God. He attacked the Almoravid religious establishment for what he regarded as anthropomorphism, for reading the divine attributes in the Quran as though God had a hand or a face or a location, and for a jurisprudence that worked from the manuals of the Maliki school rather than from the Quran and the prophetic reports directly. He also enforced the duty of commanding right and forbidding wrong with a literalism that made him unwelcome in several cities before he had any following.
Reducing this to an Arab reform arriving among Berbers gets it backwards. The Almoravids were themselves a Berber dynasty, of the Sanhaja; the Almohad movement was a Masmuda one; and the argument between them was conducted inside Amazigh North Africa, in Amazigh languages, about what Islam required.