My translation of chapter 21: What is said is not what is meant to say.
Subhuti, don't be have the thinking that Tathagata has a Dharma to teach, please don't think like this. Why so? If anybody says Tathagata has a Dharma to teach, that's defamation, and don't understand what I have said. Subhuti, the one who teaches the true Dharma has no Dharma to teach. It is called Dharma only for ease of communication.
Subhuti: World Holiness (since this is so difficult to understand), do you think in future, there will be sentient beings who listen to this Sutra and can get enlightened?
Buddha: Subhuti, (similar to what I say about Dharma) these you referred to are not "Sentient Beings" and not "Not Sentient beings". Why so? When every time I say sentient beings, I do not refer to any perceivable sentient beings. A name is given for the sake of ease of communication.
Paul's comment: The state of Enlightenment is a state free from all (constraining) concepts. When one (even Tathagata) uses concepts to describe this state of being freed from any concept, it is only logical to conclude that these concepts are not what one intends to describe! It is the same as Tao in Tao Te Ching: it is unnameable.
Subhuti's concern (expressed more than once in the Diamond Sutra) of whether there will be people in the future who are intelligent enough to understand this Sutra and can become intellectually enlightened by it! Needless to say, one who is intellectually enlightened by the Diamond Sutra still have to undergo the process of discipline, focus and wisdom (戒定慧) required in Zen Buddhism.
Question: Why did Tathagata raise the issue of indeterminacy of the concept "sentient beings" after he was "challenged" by Subhuti? One needs a sense of drama to understand the psychological interplay. Tathagata was actually saying (as a sub-text) he was not doing a factual forecast. He was telling Tathagata that he was expressing a Faith on "All sentient beings". And his Faith was based on his belief that each sentient being has a "hidden Buddha" within!