If we misunderstand ‘Buddha’ to mean something other than ‘an enlightened sentient being’ then it will be difficult to recognize the difference between ‘enlightened conduct’ and ‘deluded conduct’ (i.e. karmic activity).
As Zen/Buddhism recognizes the ceaseless-passage of reality, thus there can only be ‘enlightened conduct’ or ‘deluded conduct’ at any given moment of a particular sentient being’s existence. When a sentient being thinks, speaks, or acts inconsistently with reality as it is (i.e. Buddha-Dharma), they are an ‘ordinary being’ (deluded). When the conduct of a sentient being is in harmony with reality as it is, they are a ‘Buddha’ (enlightened). To be in harmony with the truth (Buddha-Dharma, reality, thusness, etc.) about anything here-now, be it karma, causation, meditation, compassion, is to be Buddha – to diverge from the truth here-now is to be ordinary (i.e. deluded). Accordingly, the Yui-butsu-yo-butsu fascicle opens with this point:
The Buddha-Dharma cannot be known by people. For this reason, since ancient times, no common person has realized the Buddha-Dharma and no one in the two vehicles has mastered the Buddha-Dharma. Because it is realized only by buddhas, we say that “buddhas alone, together with buddhas, are directly able perfectly to realize it.”
Yui-butsu-yo-butsu, Gudo Nishijima & Mike Cross