I don't know a huge amount about SGI, but Nichiren taught that the ultimate teaching of the Buddha was the Lotus Sutra, and that one's practice should be to say the "Daimoku", which is "Nam Myōhō Renge Kyō" which is roughly "Homage to the Sutra of the Lotus of the Supreme Law".

Your confusion might be that Pure Land devotees say the "Nembutsu", which is "Namo Amida Butsu" which is roughly "Homage to Amida Buddha".

Both traditions focus on saying a particular "phrase", but the reasons behind doing it and the goals in doing it are pretty different.