some people prefer pansexual, some people prefer bisexual, some people consider bisexual an umbrella term that includes things like pansexual & omnisexual under it. there's no polysexuality governing body that you have to go before to register and stake your claim. labels can be divisive, but they can also be a convenient shorthand for describing the gist of attraction, & for finding community or micro-community.
polysexuality of any type tends to be misunderstood and mistreated by both "sides" of monosexuality.
again, I have no quarrel with someone feeling a more personal connection with the pansexual label, my only issue with it is with misguided justifications that hinge on mischaracterizing bisexuality based on their misunderstanding.
I think it's worth noting that because [polysexual label of choice] are typically looked down on by both "sides," it's also frequently prevented them from finding community that would have helped them learn the history of their particular sexuality, so a lot of people end up figuring their shit out basically on their own, and that's part of where you get persistent misunderstandings like "bisexual means attracted to men and women only" or "... to cis men and cis women only," etc. people see a term like "bisexual" and know enough basic Latin root words to understand it means "two," but assume it must refer to genders even though the two other overarching allosexual labels (homosexual and heterosexual) refer not to a particular gender but rather the likeness of gender to which one is attracted in comparison to their own; homosexual is attraction to like gender, heterosexual is attraction to not-like gender, and bisexual is the set that contains both.
trans men are men, and trans women are women, so we don't need a separate sexuality to describe attraction to them. nonbinary/genderqueer/GNC people are genders distinct from man or woman, so I can at least understand that as an impetus for thinking a new word was needed to include them, but it's covered under like and not-like.