Reading Ouchi Seiran's "Lectures on the Platform Sutra of the Sixth Patriarch" (Part 1: Lineage - 02 Huineng's Audience with the Fifth Patriarch)

[Text of the Platform Sutra Lecture]

When the Sixth Patriarch arrived at Dongchan Temple in Huangmei and had an audience with the Fifth Patriarch Hongren, the Fifth Patriarch suddenly asked, "Where are you from, and what business do you have coming here?"
The Sixth Patriarch replied, "I am a commoner from Xinzhou in Lingnan. I have come from far away for no other purpose than to become a Buddha."
The Fifth Patriarch said, "You are a person from Lingnan, a barbarian. How could you possibly become a Buddha?" (The term 'barbarian' is used here to disparage a rustic person, similar to calling someone a savage.)
The Sixth Patriarch replied, "While there are north and south among people, there is no north or south in Buddha-nature. Even though I, a lowly person, am different from you, Master, there should be no difference in our Buddha-nature."
As Tiangui said, the forty-four characters starting from 'The Patriarch said, You are a person from Lingnan' should be based on the Jingde Chuandeng Lu. The chapter on the Fifth Patriarch in the Jingde Chuandeng Lu says:

[Supplement]

(Transcribing the exchange following 'Audience' into modern Japanese)
The Master asked, "Where do you come from?"
He replied, "Lingnan."
The Master said, "What do you seek?"
He replied, "I only seek to become a Buddha."
The Master said, "People from Lingnan have no Buddha-nature. How can you attain Buddhahood?"
He replied, "People have north and south, but how could Buddha-nature?"

[Text of the Platform Sutra Lecture]

In the world, those who study Zen often play with the koan of Zhaozhou's dog and the Buddha-nature, but I have never heard of anyone studying the 'no Buddha-nature' of Huangmei, which is a pity. The so-called Zhaozhou 'Mu' koan says, 'Does a dog have Buddha-nature or not? Zhaozhou said, "Mu (No)."' I dare to ask: does water have the nature of fire, or not? Do flowers have the nature of the moon, or not? Or, does a Buddha have the nature of a dog, or does a dog have the nature of a dog? Originally, water is water, and flowers are flowers. Why bother asking whether water has the nature of fire or whether flowers have the nature of the moon? Asking this is meddling. Rather than probing the existence or non-existence of a dog's Buddha-nature, first examine the existence or non-existence of your own human nature. The Fifth Patriarch said, 'People from Lingnan have no Buddha-nature; how can you become a Buddha?' This does not mean that only people from Lingnan lack Buddha-nature; all sentient beings in the entire universe lack Buddha-nature. Why only all sentient beings? The Buddhas of the three times and the patriarchs of all generations also lack Buddha-nature. Since neither sentient beings nor Buddhas have Buddha-nature, what do you call a Buddha, and what do you call a sentient being? Delusion and enlightenment, life and death, and nirvana are ultimately just names. If you hear that 'all sentient beings have Buddha-nature' and think that Buddha-nature is something to be realized and Buddhahood is something to be attained, you are quickly becoming two-headed and three-faced, shedding tears without wind, creating a pile of bones on flat ground, and betraying your own innate eyes and brows. If you stubbornly loathe life and death and desire nirvana, or wish to discard delusions and seek enlightenment, it is like climbing a mountain while reeling in a fishing line, or approaching an abyss while holding an axe. Unless one is a person with true insight, how can one understand the message behind the Fifth Patriarch's 'no Buddha-nature'? However, the Sixth Patriarch said, 'People have north and south, but how could Buddha-nature?' Although the Sixth Patriarch's insight is wonderful, there is still the flaw of separating people from Buddha-nature and distinguishing sentient beings from Buddhas. Is it not the case that he had not yet reached the ultimate point at that time?

[Reflections]

This is the scene where Huineng, who later becomes the Sixth Patriarch, has his first audience with the Fifth Patriarch, Hongren.
The account in the Jingde Chuandeng Lu is more concise and direct.
To the Sixth Patriarch's statement that he wants to become a Buddha, the Fifth Patriarch gives the blunt reply that 'people from Lingnan have no Buddha-nature.'
The reply that 'there is no north or south in Buddha-nature' seems wonderful, but...
In Ouchi's commentary, he notes that 'although the insight is wonderful, there is still the flaw of separating people from Buddha-nature and distinguishing sentient beings from Buddhas.' (While he transcends the dualism of north and south,) the dualism of sentient beings and Buddhas remains. He says he has not yet reached the ultimate point—that there is something insufficient.
The Zhaozhou dog koan discusses whether a dog has Buddha-nature, but the 'no Buddha-nature' in this scene is more direct because it questions whether there is Buddha-nature in human nature itself.
It likely means that one should aim for an enlightenment that transcends all dualities, such as existence and non-existence, north and south, and sentient beings and Buddhas.
Also, in the Shobogenzo, it is taught that someone as great as the Sixth Patriarch Huineng should have been questioning Buddha-nature.