My dd is 13 and she's been saying the same. As her mum, I know she doesn't really feel this way and is just wanting to fit in and create a bit of drama, but of course I just "don't get it" 
Thing is, in her group, there are maybe 10 girls and more than half of them say that they're either non binary, gender fluid or trans. I am not anti any of these things, but I just don't believe that it is now so common.
One of her friends is now a "they" and I think she wanted me to react negatively when she told me, but I didn't. The next day she said that she herself has actually always been confused about her gender (she hasn't) and if I have a problem with that, then I'm bigoted. Again, looking for some reaction from me.
Right now, I'm treading carefully. If I thought for a second she was genuinely struggling, I would support her all the way. I'm actually in the LGBT community myself and I know what it's like having truly bigoted parents, but she just wants to fit in. Nothing more, nothing less.
They're real identities, but right now, it's also a trend and I genuinely think it's becoming dangerous.
OP, I would definitely monitor what your dd is looking at online. Controversial at her age maybe, but I still would.