yeah, so definition speedrun: otherkin is identifying in some way as other-than-human. plantkin is identifying in some way as a plant. if a pronoun set (or gender, etc) are exclusive to (x)kin, it just means that only (x)kin can use them.

you’re right in a way, that otherkin isn’t on the same level as ethnicity or intersex status, and personally (as someone who is otherkin) i wouldn’t make a pronoun set or identity exclusive to otherkin (unless it was something like, a term for dragonkin who experience their gender as a dragon version of gender, which non-kin would inherently probably not be able to experience) (nonhumans in systems notwithstanding)

i guess that the coiner of bloomself had their own reasons why bloomself would be exclusive, and regardless of whether or not we agree why, we should still respect their wishes and not use them if you’re not plantkin.