It does say who they’re attracted to tho. They’re attracted to people they have an emotional bond to. There’s more sexualities than just straight, bi, and gay.
I always like to point out that orientations actually talk about who someone has the potential to become attracted to.
Gay men aren’t attracted to all men. Lesbians aren’t attracted to all women. Bi people aren’t attracted to everyone of all of the genders they’re attracted to. Pan people aren’t all attracted to everyone on the entire planet. Just to name a few.
Gay men have the potential to become attracted to men. Lesbians have the potential to become attracted to women. Pan people have the potential to become attracted to people of all genders/regardless of gender… I think you get the idea.
Demi does the same thing. Demi people have the potential to become attracted to only people they’ve already formed an emotional bond with. There’s no, “I would want to have sex with/I want to date [x celebrity],” for example, because those aren’t people you can form an emotional bond with unless you actually know them as a person. And even then, everyone you know, even everyone you know well, may not end up being someone who you have an emotional bond with.
And then on top of that, someone could identify as, to give a couple examples, demi-bisexual, or demi-homoromantic, if they want to be even more specific.
Basically, this particular exclusionist argument is built on an incorrect premise to begin with, and yes, demi-orientations do have the same function as other orientations.