People who do not conform to their gender roles are not a different gender
Some women are masculine and some men are feminine. That doesn’t mean that they are some different third gender. Just because someone does not conform to their gender roles does not mean that they are a different gender. There are plenty of “tomboys” and feminine men who are perfectly happy being the gender that they were assigned at birth. Women can feel manly and not be men, and men can feel girly and not be girls. People who identify as nonbinary are really just rejecting the gender roles of their assigned gender. This rejection of gender role does not equal a rejection of one’s assigned gender.
The existence of feminine men and masculine women does not negate the existence of nonbinary people. Nonbinary people do not just feel "girly" or "manly"; they feel that they are not men or women. Feeling like you are not exclusively a man or a woman is an entirely different phenomenon than feeling like you're a girl who doesn't want to be girly or feeling like you're a man who has some feminine qualities. Not identifying with your gender is completely different than not identifying with your gender roles. Tons of women and men reject their traditional gender roles but still identify as women and men respectively, but they are not the same people who claim to be nonbinary. A nonbinary person will generally feel uncomfortable being addressed as a man or a woman, while a masculine woman or a feminine man will not. The two phenomena are completely separate, and the existence of one does not negate the existence of the other.