I'm not aware of Nichiren expressing his thoughts on this matter.
But if we are to extrapolate?
I am assuming its contents put it in the Separate Teaching category; like the Vimalakirti Nirdesa it teaches a Pure Mind separate from phenomena. In the letter named Gift of Rice he writes:
. The meaning of the earlier sutras is that clarity of mind is like the moon, and that purity of mind is like a flower. But it is not so with the Lotus Sutra. It is the teaching that the moon itself is mind, and the flower itself is mind. You should realize from this that polished rice is not polished rice; it is life itself.