I took it as more of a comment on them being hopeful than a real shot at Batman. It wasn't that Batman could never take her out, just that they thought that whatever story he'd heard about Batman just fighting her straight-on and winning so they could too was bogus and they were screwed.
Like, it works just as well if whatever story he's referring to is true because it's not true in the way he wants it to be in that moment--iow, in a way that means they have a chance in hell of winning this fight.