Consistent with his message that his word was not received in Jerusalem, Jesus also notes that his oblique reference to other sheep was not properly understood. The discussion of what the alternative interpretation was appears to relate to post-Christ Christian interpretations of the scripture. Therefore, this might not yet have happened in the Old World, but it would. Thus, Jesus can speak of it as though it had happened, which would be less confusing.
These people are his sheep because they have directly heard his voice. That was not a privilege reserved only for the Old World.
This is not the end of a chapter in 1830. Indeed, the next chapter picks up this same theme of the “other sheep.”