With respect, I think the disconnect for you is that you're trying to generalize across a rather diverse group. There's nothing inherent to the definition of not-gender-binary that suggests "and prefers not to identify publicly" in principle.
If you imagine that gender's not a binary, then you could imagine how "trans$foo" is itself a gender. From there, some people prefer to keep it a secret. Some people prefer to make it public. A lot of times it's a matter of personal safety or privacy, but some people prefer to make their presence known.
Consider it as the cousin to coming out as gay if you like. If more folks realize that they know LGBTQ folks, eventually it's no big deal. This is well underway for LGB but not TQ.
If you imagine that gender's not a binary, then you could imagine how "trans$foo" is itself a gender. From there, some people prefer to keep it a secret. Some people prefer to make it public. A lot of times it's a matter of personal safety or privacy, but some people prefer to make their presence known.
Consider it as the cousin to coming out as gay if you like. If more folks realize that they know LGBTQ folks, eventually it's no big deal. This is well underway for LGB but not TQ.