What is recited is the title of a text, not a statement about it, and that is the whole doctrinal point.
The words are the Sino-Japanese reading of the characters Myoho-renge-kyo, which is Kumarajiva's Chinese rendering of the Sanskrit title of the Lotus Sutra, the sutra of the lotus of the wonderful dharma, prefixed by namu, homage. Nichiren's position is that the title contains the entire content of the sutra, that the sutra contains the entire content of Buddhism, and that in the degenerate age the appropriate practice is therefore the title alone. He called this the daimoku.
He proclaimed it, by the tradition's reckoning, on the twenty-eighth day of the fourth month of 1253, at a temple in what is now Chiba prefecture. That is an unusually precise founding date for a religious practice and it is preserved because Nichiren was a prolific writer who dated his own letters, a large number of which survive, some in his own hand.
He was also uncompromising to a degree that got him repeatedly exiled and once nearly executed. His position was not that the Lotus Sutra was the best of the sutras but that the others were superseded and that practising them in this age was actively harmful, and he said so to the authorities in writing, naming schools and predicting national disaster if they were not suppressed. The Mongol invasion attempts of 1274 and 1281 were taken by his followers as vindication.
The exclusivity is intrinsic to the teaching rather than incidental to its founder's temperament.