No. Natural gender means that if a noun's meaning refers to a male 'something', then the noun is masculine, and if it refers to a female 'something', then the noun is feminine.
However, not all third-declension nouns have natural gender, any more than other Latin nouns do. Sol 'sun' is masculine and feles 'cat' is feminine, regardless of the fact that the sun is not a man and a cat can be male. So I am not sure what your textbook meant.