Seeing as people here found her Hinge profile, I'm suspect she's had a few hookups with women if she says she hasn't dated any.
Or maybe she's explored it through porn or even simply flirting with a girl on Hinge.
I can imagine if she's coming to terms with her sexuality but has only ever dated men, speaking to a woman in a flirty or romantic way solidified things for her.
I think it's common for people not to believe women who aren't straight because a) patriarchy and b) women in general are accepted as being more fluid as in they'll kiss a girl "for fun", hold hands etc etc then proclaim they're completely hetero. Whereas most hetero men wouldn't even dream of doing that.
So for Sammi, maybe being flirtatious with a woman in a serious way that could have genuinely progressed into something more made her realise that yes, she could be attracted to and even get into a relationship with a woman.
I think it's common for women to say they're bicurious and we can probably all flirt for fun but chicken out when shit gets too real lol.