"Nia" is most definitely a word in the Swahili language, but that does not mean it is used as a name in Swahili culture. So the fact that it has a meaning in Swahili does not mean it can't also be a name created by African-Americans from the Swahili word. There are many other examples, like Colleen and Cherie, of English speakers creating names from words in other languages that were not used as names in the original language.