Well yes, but actually no.
The reason grammatic gender is called gender is because almost all nouns referring to men (boy, men, father, uncle…) are in one group and almost all nouns referring to women are in the other.
In German, Mädchen is not in the female group because -chen is a diminutive changing any noun’s group to neuter. The word Jungchen, from “Junge” meaning “(young) boy” exists as well and is also neuter.
Similarily, all plural nouns are in the female group. Just because grammar has some more quirks doesn’t mean grammatical gender doesn’t line up with actual gender.
The only exception in German I know of would be the word “Weib”, cognate to wife, translating to women, which is in the neuter group. Except this word is archaic and an insult nowadays. All other words referring to gendered people should be in their corresponding grammatical group.
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.
All plural nouns are not female im german.
They just happen to use “die” as their definite article when they are nominative, which doubles up as the feminine article for fem. nominative. But they by no means “change” their grammatical gender. Within the german declination system, articles are very often reused for different cases. That does never change the gender of the noun.
Just like saying “der Frau” in genitiv singular does not make Frau a masculine noun, saying “die Männer” in nominative plural does not make Männer a feminine noun.
