It isn't. If one accepts the premise that gender is separate from biological sex, one accepts chosen gender-based pronouns as acceptable descriptors of a person's gender identity, but the premise of gender-based pronouns is that they describe gender. As the set of human genders is not infinite, the set of valid pronouns to describe human gender is also not infinite.
"I identify as an attack helicopter" is obviously stated in bad faith, as "attack helecopter" is not a valid human gender. "God" or a base64 hash are also obviously not valid genders. The only people who would do so to begin with are trolls who want to mock trans identity.
Though where is the line drawn about what is a valid gender/pronoun? For instance, are all the pronouns listed on https://askanonbinary.tumblr.com/pronouns acceptable or not?
99% of this issue is about the validity of calling someone either "he" or "she" when their gender identity is not the same as their biological sex. The use of such "nounself" gender identifiers[0,1] is uncommon and controversial even with the LGBT community[2].
Personally, if I know someone is sincere and wants me to use such a pronoun, I probably would, even though I'd find it odd.
So let's draw the line here: at the very least if someone wants to be called "he" or "she" that should be respected regardless of their biological sex, and agree that it gets "complicated" from there.
"I identify as an attack helicopter" is obviously stated in bad faith, as "attack helecopter" is not a valid human gender. "God" or a base64 hash are also obviously not valid genders. The only people who would do so to begin with are trolls who want to mock trans identity.