You're not just expressing an opinion here. You're making a factual claim that lesbians don't exist and that every lesbian is actually bisexual, and the evidence doesn't support that.
Research has documented women who report being exclusively attracted to women, including population-level studies that specifically identify women who have only ever experienced sexual attraction to women. Studies also distinguish lesbian and bisexual women based on their patterns of attraction, with lesbian women generally showing more exclusive same-sex attraction than bisexual women.
The fact that sexual identity, attraction, and behavior don't always line up perfectly doesn't prove that lesbians are secretly bisexual. It means those are different dimensions of sexuality.
So “lesbian at best is a state of bisexuality” isn't an evidence-based conclusion. You're simply redefining lesbianism so that the category cannot exist, then treating that definition as proof.