I’m an old giffer and remember the 70s and 80s when there was much more “gender” flexibility. Whether male or female there was less pressure to “fit” a traditional stereotype, which is weird when you think it was 40 years ago. Since then it has polarised massively and “gender” ie some set of cultural stuff that society ascribed to your sex, has become more important - to the point where it’s normal now for lots of people to really think that “gender” is a thing they have and that it matters. It’s not, it’s just a range of sexist cultural choices that plenty of people have always resisted, while remaining their own sex.
“non-binary” means “I don’t feel I fit in with a load stereotypical societally prescribed piffle about what male or female people should be like”. In other words the same thing many thinking, questioning people have realised since the year 0. It’s just that in the past we rightly didn’t take gender so seriously so it didn’t matter - in the 80a only the most boring, unimaginative people would have thought that you should adhere to gender norms.
The interesting thing was also that breaking gender norms didn’t affect your sex, which remained a valid part of you. A man could wear a ton of make up and have long dyed hair and be wildly attractive to straight women and very masculine in his own way, like nick Rhodes . Or do the same and be a gay man, like boy George. Going back even further that was also true, eg reformation styles for men.
To be “non-binary” you have to perceive a strict binary to fall in between. It’s not really there and never has been.