Im sorry, but what this person told you is untrue though. A noun retains its gender in whatever declination it is, however, articles often double up within the german declination system. And it just happens to be that the plural definite article for nouns of all genders is "die", which is also the nominative singular definite article for feminine nouns. But the article never determines the gender of the noun, the gender of the noun determines which article to use. It's just sometimes, the article looks like looks like another article already used for something else. That's why every school book and german learning course tells you to put the noun in nominative singular before trying to tell the gender.