That's because it isn't a real thing.
What the Tumblr crowd calls demisexuals are really just heterosexuals, bisexuals, and homosexuals who prefer to be romantically attached before having sex. That being said, I suppose people can self-identity however they like.
Well, that's big of you?
I'm curious, if you're fine with people self-identifying as they like, then why do you feel prompted to couple that with "..but it's not real"? If it's a meaningful distinction for them, why do you care? If it's not useful to enough people, it will go away on its own, or stay restricted to a small group.
For what it's worth, I have a friend who is demisexual,and it wasn't until I realized that's what she was (and she confirmed it after hearing a definition) that I understood how much of a difference it really was. She was absolutely not attracted to people or interested in dating to the point that we assumed she was asexual (as much as we could understand those things, this was many years ago and we weren't really that informed), until she met someone and started dating him and it blew our minds. And her experience of sexual attraction in their relationship is markedly different than my experience of it in mine, even though she still has it. And furthermore, once I supplied her with that definition, she was floored because she had felt "different' /"wrong" her whole life because she experienced these things so differently. Having that as a concept was very validating for her, and has now allowed her to explain it to people better, and I understand her better too. I don't care where the term came from, or how many people identify as it, or if there are some people who are "just using for special snowflake status", because I have seen how it can help people make sense of their lives in meaningful ways.