It's not about when movement started, things need to change or accept that they are no longer relevant. Feminism is about equal rights for women but now that there is a legitimate voice for other rights for men (seperate but equal) Humanism is what Feminism claims to be whilst Feminism by its nature still prioritizes women over men. Some would say that's the way things should be and I agree because I think that's what our true nature is - women and children first. But don't call it equality for everyone because it's not. That's my point. Not "they were named wrong back in the day".
EDIT: I just re-read your post to see if maybe I misinterpreted but you really did say my argument wouldn't hold up to the 1800s so doesn't hold up today. JFC.
maybe i said it weird. let me rephrase it
my argument is
feminism as a movement started in the late 1800s
the movement has persisted since then
you can't really "rename" a movement or set of ideals unless there's some massive cultural dealignment against it which hasn't happened
so while the name "feminism" may imply favoring women, this is really just a consequence of it being named 100+ years ago when women had substantially less rights and the movement could justifiably be named such