It makes sense when you realise that two-spirit isn't what two-spirit people call themselves at home. In their own tribe's cultural context, everybody knows the specific name for their gender and what it culturally means. Two-spirit was invented on purpose as a shorthand for describing dozens or hundreds of different things in a language people outside that one single tribe can all understand. It's as manufactured and as broad as the sequence of letters "LGBT", an umbrella term that arose during a specific cultural moment as part of a push for equality.