have you never seen the Antifa flag and what it's modeled after?
Even now, I'd say the majority of the 'members' of this movement, as in the people that show up, are all sorts of people, generally left-wing but including what in the US would be called 'liberals'. Most of these are not explicitly members of AFA.
So I'd say 'co-opted' sort of doesn't fully cover the situation. But I'd also say you're not entirely wrong either.