It's more complicated than that (but just as stupid).
The term "anti" (short for "anti-shipper") started as a term for people who just didn't like a specific ship originally (like, say, someone who didn't like people shipping Mulder and Scully), but that usage was just standard fan discourse -- not a moral judgement. Like you'd casually drop that you were anti-a specific ship just because you didn't like it, not because you thought the people shipping it were morally wrong for doing so.
Think of it like how people would root for a sports team. Standard fandom rivalry stuff.
The modern usage really joined the party in 2016 though -- right here on Tumblr largely. And you're right, it was for people who were anti certain kinds of darker ships. But unlike the "antis" of the past, this wasn't about not wanting to read those things, it was about the existence of people who enjoyed those ships being morally wrong as well.
And what qualified as "problematic" kept expanding with those people. It sounds reasonable to most people when someone says they're anti-pedophilia in the fiction they consume. But then you find out they consider a story where a person is depicted as a child at some point in fiction means any adult romantic pairing must be "pedophilic." Or that a character is "child coded" because they're short. Or that a three year age gap between adults is "grooming." Literally everything
And, again, if this was just people choosing not to engage with those stories, no one would care. Like if exploring darker stuff bothers you, you should definitely not consume that kind of content. But instead they've gone after fanfic writers, websites, and people who enjoy it.
They've literally contacted the FBI about AO3.
Antis, fundamentally, can't differentiate between fiction and reality. They believe the fiction someone consumes reflects actions in the real world. Like they believe that everyone who consumes non-con stories wants to rape people (never mind that some survivors use the topic to explore their trauma). It's a fundamentally deranged way to look at the world, and used as a justification for harassment and censorship.
So, people getting frustrated with these "antis/anti-shippers," who wanted to go back to the traditional "ship and let ship" philosophy that fandom spaces were built on, decided to give themselves a name. At first it was "anti antis," but later settled on the inverse of "anti-shipper" with "pro-shipper."
But, of course, the "antis" need to cast anyone who idealogically opposes them as a bad person, so you can imagine that things just get worse from here.
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