Lemmy.world pre emptively defederated them before hexbear federated with wider lemmy because hexbear's a very active community of communists who've been on an undefederated fork of Lemmy for three years or so.

Hexbear users are very active in calling out anything they perceive as reactionary thought (transphobia, racism, ableism, homophobia, support for imperialism, colonialist apologia, etcetera) and pretty frequently get hostile about it, one of the stated intentions of them federating was to bully bigots into silence in order to make all of lemmy a more left wing space. Hexbear has a broader definition of what constitutes as reactionary thought than most people do though so they'll frequently attack liberals too.

Basically they felt hexbear's community wouldn't mesh well with lemmy.world