Well, I don't call myself lemming, oh wait I actually do sometimes but it has nothing to do with lemmy, yeah, I digress. Yet "lemming" is for sure less confusing than "Fedier", how do you even pronounce it?
I'd say that any variation of Fed would apply to the entire fediverse, not just lemmy or kbin. So Mastodon users, Pixelfed users etc would also be Fedier.