Zen can be defined several different ways, but one would be the unmistakable, handed-down tradition Master Dogen described in Bendowa. This concept of teacher-to-student transmission, with a lineage traced back to the Buddha himself, is an integral, if not defining, aspect of Zen, and to imagine "zen" without transmission would be to imagine something other than Zen.
But the lineage itself is a hoax, an artifact of a time in Chinese history when various Buddhist schools were vying for prominence and recognition. A group of contemplative monks claimed that they were the one school practicing the true teaching of the Buddha, and to back up their claim, they manufactured a narrative of students receiving the true teaching from their teachers, and claimed the chain extended all the way back to the Buddha, In order to flesh out the claim, they had to fill in the occasional historical gap by inventing "transmission stories" for some teachers of whom very little was known. These collections of stories became known as the "lamp" anthologies, "lamp" used in the sense of the passing on of the flame. Today, and at least since the time of those Seventh Century monks, records of the teacher-to-student transmission have been carefully maintained.
But this very narrow and exclusive lineage manages to leave out all of the other great Buddhist traditions, and implies that the Tibetan Buddhists, the Theravadan monks of southeast Asia, and even the other Chinese traditions all were practicing an inauthentic teaching. This very narrow and exclusive lineage manages to leave out all of the other teachings in the world, from the Dao of Lao Tzu to the transcendentalism of the Christ, from the Greek philosophers to the European Enlightenment. Meanwhile, I have little doubt that were I to research these other traditions, I would find that they each claimed to have the one true teaching.
This isn't to dismiss Zen or Zen teachings at all, but just to assert that perhaps there may be wisdom, there may be insight, there may even be realization, in practioners outside of the transmitted lineage. Further, I suspect that in Western culture especially, with its fetishistic attachment to credentials and the power of its authority figures, the almost fundamentalist concept that only the transmitted can be teachers or be awakened is especially strong.
I challenge that notion.