"Earlier, you quoted a passage from the Daibontenno Mombutsu Ketsugi Sutra to prove your contention that Zen is 'a separate transmission outside the sutras.' But by quoting a sutra passage you were already contradicting your own assertion. Moreover, this sutra represents the provisional teachings, and, in addition, it is not listed either in the K'ai-yuan or the Chen-yuan era catalogues of Buddhist works. Thus we see that it is a work unlisted in the catalogues and a provisional teaching as well. Hence the scholars of our time do not refer to it; it cannot be used to prove anything." -- Nichiren Daishonin
Likewise, those writings of Nichiren not found in the Rokunai or Rokugai catalogs*
http://markrogow.blogspot.com/2014/11/the-roku-nai-list-of-authentic-gosho.html -- http://nichirenscoffeehouse.net/gosho.html
can not be used to prove anything.
*The Rokunai Gosho catalog was collated by Toki Jonin shortly after the Daishonin's death and the Rokugai by various disciples within a hundred years of his death.