Near as I can tell, the basic point is that gender based discrimination and prejudice needs to be applied across the board.

Feminism focuses on the needs of women, and works to bringing women to an equal footing in all aspects of society. By nature of that effort, that doesn't include men's issues since social and cultural norms already have men at the standard of what we're all supposed to have.

However, there are still plenty of issues where prejudice, stereotypes, and harmful norms affect men. Anti-sexism serves to also work against those, as well as the issues of anyone that doesn't match the man/woman binary.

So, feminism would be an important aspect of anti-sexism, but not the entirety.

See, feminism does result in outcomes that benefit men as well, but that's a side benefit rather than the goal. With anti-sexism, it is a goal as important as any other. Since there's a synergistic effect, I might even argue that anti-sexism efforts boost feminism in a way that it can't reach on its own just by virtue of not having to be held back by slower progress in men experiencing the freedom that feminist thinking gives.

Broadly speaking, Feminism does mean to oppose sexism and level the field between men and women. There are of course going to be people, overwhelmingly men, who are opposed to that in the first place, but we don't care about their opinions right now.

Unfortunately, some people have taken to using the label of Feminism not to advocate for women, but to attack men. This comes in both the form of denigrating men as a whole and also attempting to erode their rights specifically. I don't think it's an especially large group, but rather is fairly vocal by nature and their message gets amplified by men eager to feel like they're actually the victims. Either way, it does somewhat taint the well of "Feminism" when a portion of it's purpose seems to just flip the oppression, rather than eliminate it.

Anti-sexism then is a more neutral term that doesn't have that "Us vs Them" mentality baked into it. It also has the added benefit of including advocating for those who don't strictly define themselves as Men or Women, being a term that opposed sex-based discrimination of all forms. Ideally, Feminism and anti-sexism would be basically synonymous, and in most use cases it would be understood to be. When presented side by side, though, the finer details will stand out more.

Hello from All - I can't speak on behalf of the creators of this community, but I'd guess it's a branding kind of thing.

born from a misunderstanding of feminism as being misandrist

That's a fairly common misunderstanding. For some people, the error is easy enough to explain away; others will be so bothered by the 'fem' part that there is literally no convincing them that feminism is NOT anti-male. Personally I consider those people to be a lost cause and not worth the energy, but if you wanted to sway them, your best bet would be to advocate for the concept of feminism without actually saying the name: then maybe you'll see some positive behavior change in things like voting patterns, all without needing to tackle the Sisyphean task of coaxing them outside of their comfort zone of ignorance or bigotry.

If an anti-feminist could be swayed into being anti-sexist, even if the fact that 'feminist' and 'anti-sexist' are synonymous is over their head, that's a win for human rights. ...even if the route to getting there is kinda stupid.

'a rose by any other name...'

Like any word "feminism" is subject to fluctuating definitions. Historically, and currently on the academic level, feminism is indeed synonymous with anti-sexism.

However there is a wide range of people who use it to mean misandry. The gender wars narrative is becoming massive in the society.

The male side of it is widely spoken about, represented by Andrew Tate and such, but there is also a female side who unfortunately appropriated the name of feminism.

For a while I used Tiktok, and half of my feed was stuff like "men are useless emotionless parasites". I was definitely served that as algorithmic ragebait, but I wonder how many actually think like that.