Maybe in your world. In the LGBTQ realm it is more nuanced and not nearly so black-and-white.
Girls who date girls and s___p with girls are lesbians. They may also be into guys, which makes them bi, but that does not disqualify their lesbian tendencies.
Additionally, prior interest in guys does not exclude one from being lesbian. If so, according to your strict standards every woman who admitted her interest in girls after she divorced a guy could not call herself lesbian because she had previously been with guys.
Consider this analogy:
If someone determines that she will only eat plant-based foods she calls herself a vegetarian. That label (for whatever value it holds) does not consider that two years ago she may have worked at McDonald’s and ate Big Macs for dinner five nights a week. Or someone who marries a Jew and converts to Judaism may now keep kosher even though she previously loved shrimp and lobster sauce.