Weeellll I hate to nitpick but "they" only has a nonbinary connotation because of the hype around the issue atm, but it has been used for a good long while to refer to people of unknown gender (or a plurality thereof). The best example I can come up with is think about electronic music, Major Lazer, are they a group? One dude? One woman? One of each? Two of one and one of another? Who knows what they are (shhh you could google it but ykwim). "They" is the natural choice in this instance, otherwise you're just guessing.

Also it does happen to fit in with the nonbinary crowd for much of the same reason, many of them want to appear androgynous or of unknown gender and so they basically use it on the same principle but it isn't exclusive to them nor should it necessarily conjure up images of nonbinary people just because that's currently the new use, it's probable many people have been using it as I described and never even noticed because it was just ingrained, and we only notice it at the "new" application regarding a specific individual's gender that we may perceive as "known" subconsciously.