The four-line verse that came to define Chan, and later Zen, is not as old as the tradition claimed for it: a special transmission outside the scriptures, not depending on words and letters, pointing directly at the human mind, seeing one's nature and becoming a Buddha. It states a claim about how awakening is conveyed, from mind to mind through a lineage of masters beginning with the Buddha's wordless holding up of a flower, understood by Mahakasyapa alone.
What is being rejected is not learning but a substitution. Doctrinal mastery, however extensive, is a description of the terrain and not the journey; a person can know the analysis of the aggregates perfectly and remain wholly bound. The tradition's slogans, the finger pointing at the moon, the raft to be abandoned after crossing, all press the same distinction.
The irony is deliberate and the tradition knows it. Zen produced one of the largest literatures in Buddhism, a whole genre of recorded sayings, koan collections with layers of commentary, and monastic codes of great precision. Its masters were typically formidably read. The claim was never that texts are worthless, but that they cannot do the one thing that matters.
The sudden and gradual distinction was formalised in an eighth-century controversy. The account that prevailed, set out in the Platform Sutra, presents a Northern school teaching gradual purification, likened to polishing a mirror, and a Southern school under Huineng, an illiterate layman, teaching that the nature is already pure and is seen all at once. Modern scholarship regards the Northern school as a polemical construction, and its actual teachers as less gradualist than the story requires, but the story shaped everything that followed.