I would be curious to see actual numbers on these groups

Between 7-10% of women say they are mostly heterosexual so the total number of bisexual women isn’t super tiny in my opinion if we count the whole range of bisexual women, and a mostly heterosexual bisexual woman would still be able to get something out of a same-sex relationship. We might have different preconceptions of how many strictly straight women there are who would be willing to be in same-sex relationships though, which is fair.

Yeah, but she could just be lying, since she lies about the nature of sexual orientation.

I think there is difference between intentionally saying something you know is false and being wrong (due to biases or not). I think Julie Bindel is wrong about the nature of sexual orientation, and if she is bisexual it would make sense that she is wrong if she is generalizing from one example (herself) and thinks everyone is capable to be attracted to both sexes because she is. In her arguments that is what her assumption seems to be, that everyone is bi but can choose to be behaviorally homosexual.

she claims she is lesbian, lol, not bisexual.

She uses lesbian as synonym to political lesbian, someone who is attracted to women and can be attracted to men but chooses to be behaviorally homosexual (as she has criticized women who have same-sex relationships just to make a point it seems like she thinks attraction to women is important too), that is what we call febfem. That is not the same thing as claiming to be a female person who only has the capacity to be attracted to the same sex, she specifically claims that isn't the case for her. So translated to our words she is claiming to be female exclusive bisexual and not a lesbian but she wants lesbian to be the word for febfem (as she doesn't think homosexuality exists).