The existence of intersex people disproves the gender binary

Technically, the existence of intersex people only proves that SEX is not a binary thing. GENDER is an entirely different concept. I mean, yes, gender is even less binary than sex (if there is such a thing as a measure of how binary something is... I like the irony 😋), but technically intersex doesn't say anything about gender. 😉

True, i was just commenting on how people conceptualize gender as two opposite categories of either male or female, basing said categories on anatomy. for that reason, i was saying that the presence of individuals with ambiguous and/or "atypical" genitalia/chromosomes supports why the gender binary fails to encompass everyone's identity :)

Could someone please explain intersex to me

Intersex is basically the third sex. Intersex people might have both male and female genitals, or their genitalia might be something between those two.