"Biphobia" is a bit different then I think you're portraying it. There are specific stereotypes about bi people. One stereotype is that bi people don't really exist. They're either gays afraid to come out of the closet or they're straight and faking it for clout/attention. Another is that bi people are all just slutty. A third is that bi people can't commit to monogamous relationships because a single person can never fulfill a bi person's needs.

These are specific experiences that I think can be called biphobic. They also are, like all stereotypes, stereotypes that have some grounding in real world truths, which make them complicated to talk about.

I also think it's easy for people to feel like bi people with no significant same-sex relationship history should opt out of claiming political space or awards in LG+ communities, but much harder to figure out how to incorporate the specific experiences of people who have had significant same-sex relationships but are now in straight relationships into the broader LG community. For example, Chirlaine de Blasio was an out lesbian for over a decade before she married Mayor de Blasio. Of course, all of those experiences aren't simply erased for her now.

It's much easier to attack Greenwald as biphobic than to actually think through the complexities, especially if it may logically lead to admitting that a lot of bi people really have minimal understanding of LG lives.