I've heard of extremely rare cases where it didn't (like once in a billion) so I wanted to cover it all.

It still was, there was just misunderstanding, as doctors were not qualified enough for this.

However, if speak about ALL cases, then true - there are such cases, but they all deadly, childs with it are dying before being born, they can't develop as both male and female or as neither - it is incompatible with being alive in humans.

I am ESL (English second language) so I am not always familiar what is considered outdated terminology in English.

Neither do I. I just know personally person with intersex condition. Or more like, I learned she has such condition only when TRA started misusing her condition and she complained about it, and especially about stuff like in NZ - when they can be denied vital healthcare because now it is not medical condition but just "identity". Before that we never talked about it.

Also, my language never even had such term until last few years, when LGBT organizations from the USA started promoting homophobia and gender ideology. I think in my language it is called something like "group of congenital disorders of sexual development". And they all are not even grouped together, as there are like 3-4 different groups in them.