Shobogenzo Dōtoku X: Further Close Shaves with the Truth.

Master Dogen discusses the koan regarding Seppō and the 'hut master':

[204] This episode is truly like an appearance of the uḍumbara. It is not only
difficult to meet, it may be difficult even to hear. It is beyond the scope of
[bodhisattvas in] the seven sacred stages or ten sacred stages and is not
glimpsed by [bodhisattvas in] the three clever stages or seven clever stages.
Sutra teachers and commentary teachers, and adherents of mystical powers and
apparitions, cannot fathom it at all. “To meet the Buddha’s appearance in the
world” means to hear a story like this.

The uḍumbara is a mythical flower said to bloom only once in an age. Master Dogen presents this account as being beyond knowledge of sutras, the attainment of mystical powers, and all the traditional, gradual schemes of realisation.

Now, what might be the meaning of Seppō’s “Express the truth and I will not
shave your head.” When people who have never expressed the truth hear this,
those with ability may be startled and doubting and those without ability will
be dumbfounded. [Seppō] does not ask about “buddha,” he does not discuss “the
Way,” he does not ask about “samādhi,” and he does not discuss “dhāraṇī.”

Master Seppō doesn't talk in abstract Buddhist terms but is very practical and direct in his expression and his conduct generally.

Inquiry like his, while seeming to be a request, also seems to be an assertion.
We should research this in detail. The hut master, though, because of his
genuineness, is aided and abetted by the expression of the truth itself and is
not dumb founded. Showing the traditional style, he washes his head and comes
forward. This is a Dharma standard at which not even the Buddha’s own wisdom can
arrive. It may be described as “manifestation of the body,” as “preaching of the
Dharma,” as “saving of the living,” and as “washing the head and coming
forward.”

Seppō's inquiry seems like a question, but it is also a statement of the 'koan' of reality. Can we ever express reality directly in words and keep our flowing locks? The hut master recognises the truth in this question/statement and responds by coming forward. Master Dogen presents this straightforward expression as the essence of Buddhism.

Then, if Seppō were not the real person he is, he might throw down the razor and
roar with laughter. But because Seppō has real power and is a real person, he
just shaves the hut master’s head at once.

Master Dogen admires Seppō for following through and actually shaving the hut master's head. Seppō wasn't talking in abstract, philosophical terms or engaging in 'koan-speak': He was talking about really shaving the guy's head!