It's not exactly new. They/them has long been a recognised pronoun for a person whose gender is unknown/unspecified in the speaker's frame of reference. (Ditto when number is not known, so the subject may be either singular or plural.) I assume that is what this form of usage derives from, and it's hardly a quantum leap. If a person wished to identify as nonbinary/gender-fluid, it makes a lot of sense to me, although I'm not prescribing it as such as I'm sure everyone who chooses it has their* own rationale.
*See? Perfectly grammatical and normative, and typed it without even thinking about it!