It has nothing to do with whether they were fully human, it has to do with whether they were ever regarded as a member of the nation.
The root word of "Nation" is "Nat", from which we get words like Natal and Nativity, and it's meaning revolves around birth and nature.
A "Nation" for most of human existence, was mostly a group of genetically similar people that lived in proximity to each other. It was in essence, an extension of the Tribe or Family from which the larger group sprang.
Adoption into the family or tribe could occur, but the normal method was to be born into it.
Members of the Nation-tribe could recognize the humanity of members of other Nation-tribes, but this was not the criteria by which they would be recognized as members of their own Tribe. Unless they were adopted into the tribe, they were regarded as "Alien."
I came in late, and ASSUMED that the discussion was about a black person in the US NOT being considered an NBC.
Since they weren’t considered fully human, they weren’t considered even part of the discussion.
So, any effort to derive an NBC definition based on the treatment of blacks in America is a dead end from the beginning. They were irrelevant to the discussion at that time.