Obligatory video; didn't watch, but biological sex is medical/scientific term, not a social one. The fact that some people are intersex doesn't detract from that, it's just part of the medical definition. Also, it's not a binary - as I just mentioned, intersex people exist and have a scientifically defined biological sex that is not in the binary of male or female.
Gender is a totally different thing, it is a social construct and also very much not a binary.