It's not a thing in the sense of it being some innate part of the universe. But it's very real in the sense of societal expectations. It's all our natural differences, but Neurotypical is just the acceptable amount of difference that society is built around and caters to. If you fall outside of that you're neurodivergent. Of course people aren't sorted into playing life on easy mode; their level of difference simply naturally falls in that acceptable range. They don't have to think about it like someone else might because it just comes naturally to them. It's considered a disability because everyone is expected to operate inside of that acceptable range and if you can't then you're broken according to society.