Prior to Nichiren's time, reading the Lotus Sutra in its entirety was said to result in benefit, or good effects, in this life and the next. As part of Nichiren's efforts toward his goal of "saving all people" as he put it, he said that reciting the title (prefaced by the Sanskrit word Nam) was enough to gain the same benefit. It is in this context that Nichiren wrote. Here is a quote from the same letter (gosho), The One Essential Phrase, where he provides a rationale. As can be seen, he refers to earlier writers in support of his teaching.
The spirit within one’s body of five or six feet may appear in just one’s face, which is only a foot long, and the spirit within one’s face may appear in just one’s eyes, which are only an inch across. Included within the two characters representing Japan is all that is within the country’s sixty-six provinces: the people and the animals, the rice paddies and the other fields, those of high and low status, the nobles and the commoners, the seven kinds of treasures and all the other precious gems. Similarly, included within the title, or daimoku, of Nam-myoho-renge-kyo is the entire sutra consisting of all eight volumes, twenty-eight chapters, and 69,384 characters, without the omission of a single character. Concerning this, Po Chü-i stated that the title is to the sutra as the eyes are to the Buddha. In the eighth volume of his Annotations on “The Words and Phrases of the Lotus Sutra,” Miao-lo states, “When for the sake of brevity one mentions only the daimoku, or title, the entire sutra is by implication included therein.” By this he means that, although for the sake of brevity only the title of the sutra is spoken, the entire sutra is contained in the title alone.
Everything has its essential point, and the heart of the Lotus Sutra is its title, or the daimoku, of Nam-myoho-renge-kyo. Truly, if you chant this in the morning and evening, you are correctly reading the entire Lotus Sutra.
And another, similar, quote from a different letter, Expedient Means and “Life Span” Chapters -
First of all, when it comes to the Lotus Sutra, you should understand that, whether one recites all eight volumes, or only one volume, one chapter, one verse, one phrase, or simply the daimoku, or title, the blessings are the same. It is like the water of the great ocean, a single drop of which contains water from all the countless streams and rivers, or like the wish-granting jewel, which, though only a single jewel, can shower all kinds of treasures upon the wisher. And the same is true of a hundred, a thousand, ten thousand, or a million such drops of water or such jewels. A single character of the Lotus Sutra is like such a drop of water or such a jewel, and the hundred million characters are like a hundred million such drops or jewels.
(the quotes were added in response to a request in the comments.)