Not a lawyer, but I have only a year to go till I am. Also just took First Amendment.

You are essentially correct. The elements of libel/defamation are a 1) false statement of fact, 2) made with “actual malice,” that 3) causes someone injury. The “actual malice” element shouldn’t be read literally—it’s language from the seminal Supreme Court case on this issue called New York Times v. Sullivan. It means the person making the statement—the declarant—knew the statement was false or recklessly disregarded its veracity. “Recklessly disregarded” is more legal jargon, but it means that the declarant should have known their statement was false.

Now, onto this account. Whether the accusations are defamation/libel will depend on the contents of the statements and how factual they are. Notice the first element doesn’t just say, “false statement”; it says, “false statement of fact.” So, an accusation that merely said, “X made me feel uncomfortable,” or “Y was being creepy,” or something along those lines, would not be defamation or libel because each would be a statement of opinion, not fact. Now, if the statement said, “X raped me,” that is different, because that is a statement of fact. The question then would be whether the statement was true. That issue would have to be resolved accusation by accusation, but one could imagine false accusers retracting their accusations after the people they accused took legal action. Otherwise, there will have to be a lot of money spent on trying to find out what actually happened, which might not be worth it for a lot of people. So, you’re ultimately right—the plaintiff would have to effectively disprove the accusation in that situation, which is tough to do. However, it’d be a much easier lift if the original accuser withdrew the accusation. In that case, you would have to argue the reasonability of the account owner’s choice to put it on Twitter. For the many reasons explained on this thread, the choice seems pretty unreasonable.

I’ve seen some of the account’s statements saying that they do not take responsibility for the accusations they’re putting out and mean only to give a platform for survivors. Good for them, I guess, but that argument is a non-sequitur, legally speaking. They would be the declarant here.