[Clarification, since people in the comments seem to have misunderstood: this is not about literally every identity ever. This is about identities that people have because they self-identify as them, and how they can still totally have a definition.]
A lot of people tend to say things of the form “if you identify as an X you’re an X.” If you say you’re a man, you’re a man; if you say you’re a woman, you’re a woman. If you say you’re a geek, you’re a geek. If you say you’re a feminist, then you’re a feminist. The problem with this is that it makes the word ultimately meaningless: if “geek” only means “the set of people who say that they’re geeks,” then “geek” doesn’t mean anything at all. You might as well say “fleegash means people who say that they’re fleegash.”
So let’s not talk about gender or subculture. Let’s talk about Hogwarts houses.
Unfortunately the real world lacks a Sorting Hat, so we can’t say for sure what someone’s Hogwarts house is. The ultimate authority about whether someone’s a Ravenclaw is their own word that they’re a Ravenclaw. (You could say that Pottermore tells you “canonically” what your house is, I guess. If this really bothers you, you can pretend that I’m solely talking about pre-Pottermore fandom.)
If someone loves reading, knowledge, and being snotty about how much smarter they are than everyone else, and identifies as a Gryffindor, they’re probably a damn Gryffindor. I mean, to you they look like a Ravenclaw, but they have privileged information here. They know about the time they fought off bullies in fifth grade and how they’re only learning things in preparation to become a Warrior of Light. Unless they tell you literally every damn thing that happened in their lives, they know more about themselves than you do. And that kind of information is important for a situation as full of nuance and shades of grey and judgment calls as fitting your entire personality into one of four arbitrary boxes– Hero, Villain, NPC, or Other NPC.
Yes, it will occasionally happen that someone is mistaken about what house they’re in. But from the outside you can’t tell apart “mistaken” and “knows more about themselves than you do.” And since people are only sometimes mistaken and always know more about themselves than you do, and since being like “no, you are CLEARLY A RAVENCLAW!” is generally a poor method of convincing them that they’re mistaken, you should err on the side of respecting what people say. After all, what’s the worst that could happen? Your clubhouse gets invaded?
So, yes, you’re a Ravenclaw if you identify as a Ravenclaw. And yet the Houses have clearly defined traits: Ravenclaws are smart, Slytherins are the villains, Gryffindors rush in where fools fear to tread, and Hufflepuff… is near the kitchens. Both of these things can be true at the same time! And the same thing is true of feminist, geek, man, woman, nonbinary, and anything else people say “if you identify as an X you’re an X” about.