What it definitely doesn't include by definition of the word is people who are totally outside of the binary spectrum.

It does though.

This is a quote from the bisexual manifesto, back from 1990

Bisexuality is a whole, fluid identity. Do not assume that bisexuality is binary or duogamous in nature: that we have “two” sides or that we must be involved simultaneously with both genders to be fulfilled human beings. In fact, don’t assume that there are only two genders. Do not mistake our fluidity for confusion, irresponsibility, or an inability to commit. Do not equate promiscuity, infidelity, or unsafe sexual behavior with bisexuality. Those are human traits that cross all sexual orientations. Nothing should be assumed about anyone’s sexuality, including your own.

I completely understand that you may be uncomfortable with the term yourself, and I'm not suggesting that you need to use it. But the term was inclusive of non binary people from before many people using the internet today were even born. You can't assume that someone using the term is exclusive of non binary folk.

I can genuinely say that I've never met a bisexual person who is explicitly only interested in men and women. I mean, I've ran across them online, but the people that I've actually met and spoken to in person? Not a single one has used the label in an exclusionary way.

And like any term with problematic, out of date origins, there is power in reclaiming it.

All of which is to say, you can't tell people that an identity they've been using in an inclusive way for literally decades is actually exclusive just because you personally aren't comfortable with it.

For what it's worth, I feel similar about the term transsexual. It's a term that in modern usage, has a good chance of meaning that the person labelling themselves that way is a transmedicalist, with exclusionary beliefs about who is and isn't transgender. I don't label myself transsexual because of that discomfort with the word. But I also know people who came out as trans decades before I did, who use the label because that was the language at the time they came out. They're not automatically transmeds themselves, and I don't get to tell them that they need to redefine their identity for my comfort.