Sigh. No, not everyone is a little bit nonbinary, or a little bit bisexual, or a little bit autistic. These labels either fit who you are and are helpful to describe you and your actions, or, they don't fit you. It's not like people are walking around like The Sims with a little "5% percent bisexual" hovering over their heads.
Sometimes a person saying, "isn't everyone a little bit...?" is a closeted nonbinary person or bisexual or undiagnosed autistic person. They relate to the experience you share and assume everyone feels that way. People tend to cluster in similar friend groups or communities so if you hang out with a bunch of nonbinary people, for example, you may be more likely to assume everyone is nonbinary. But being nonbinary is not a universal human experience.
The more pernicious argument comes from someone who does not want you to use labels for your identity that differ from the norm. "Gender is on a spectrum, and everyone is somewhere on that spectrum, so why do you need to call yourself nonbinary? Everyone is nonbinary, but not everyone talks about it."
By saying your label fits everyone, they are trying to remove all meaning from the label so you stop using it. Usually, calling yourself a "woman" or "man" or "straight" is no problem. It's only labels that differ from our cishet colonial culture that these people object to.
The most perplexing conversation I've had about labels went something like this:
They said, "I believe everyone is bisexual. We would all be bisexual if not for cultural norms."
I asked, "Are you bisexual, then?"
They replied, "No, definitely not. I'm straight."
