Zen understands itself as a transmission rather than as a doctrine. The formula, itself a later composition, describes a special transmission outside the scriptures, not founded on words and letters, pointing directly at the human mind. Authority in Zen is therefore neither textual nor conciliar. It passes from a teacher to a student who has been recognised, and the record of that recognition, Dharma transmission, is what makes someone a teacher.
That makes the lineage the institution. It also makes the historical standing of the lineage the tradition's permanent live question, and the honest answer is that the chain as the tradition draws it is a retrospective construction.
Bodhidharma, the Indian monk who by tradition brought the teaching to China, is a real but very thinly documented figure, and the three early sources disagree about him. A Luoyang record of 547 calls him a Persian Central Asian. The preface by his disciple Tanlin calls him a South Indian, the third son of a king. Daoxuan, in the seventh century, says South Indian brahmin stock, arriving before 479. The famous material (nine years facing a wall, the exchange with Emperor Wu, the single sandal), appears in a collection of 952, some four centuries after his supposed lifetime.
The succession of patriarchs is later still. John McRae's work established that no lineage theory existed in the earliest Chan and that the chart was assembled afterwards, in the course of a contest over legitimacy; Shenhui, in the eighth century, campaigned Huineng into the position of sixth patriarch. None of this makes Zen less interesting. It makes it a tradition whose founding document is an argument about who owned it.