Yeah but the difference is there is no dictionary definition that supports that definition of feminism. It’s just an interpretation. You could say the same thing about veganism as being a philosophy that animals are superior to people. That’s a more direct metaphor and you would be just as wrong, and no dictionary would agree with you.

Any “ism” will have multiple ways to define it, and those who coin a term don’t get to define its evolution. If you want to take the “GNU/Linux” approach and insist everyone else is using the word wrong then go for it. But in modern parlance, “vegan” is frequently if not almost always used to refer to the diet, whereas “veganism” does evoke the stricter definition you’re touting.