Snopes says this was mostly false, of course.
Opposing fascism or wearing an “Antifascist All Stars” T-shirt does not necessarily make one a member or supporter of “Antifa,” a frequently misunderstood and misused term that describes a movement of typically militant and sometimes violent left-wing activists in the United States and beyond.
Despite those nuances, Thunberg appears to have received criticism from some quarters for her decision to wear the T-shirt, and she eventually deleted the original photograph from her social media account. In a subsequent tweet, she offered an explanation that made it clear she had intended only to express her opposition to fascism, and that she rejected the use of violence:
Yesterday I posted a photo wearing a borrowed T-shirt that says I’m against fascism. That T-shirt can apparently to some be linked to a violent movement. I don’t support any form of violence and to avoid misunderstandings I’ve deleted the post. And of course I am against fascism.
— Greta Thunberg (@GretaThunberg) July 26, 2019
Given the inherent ambiguity of meaning in wearing the T-shirt, as well as Thunberg’s subsequent explanation and deletion of her tweet, it is difficult to support the characterization of the photograph contained in the meme, which claimed Thunberg had “aligned” herself with Antifa.
https://www.snopes.com/fact-check/greta-thunberg-soros-isis-antifa/