It says that anyone could propose a candidate, and that the person elected in that specific election wasn’t part of the [Communist] Party, making it somewhat likely they weren’t a communist.
But a better question, is why is it important that they can vote for non-communists? What else should they vote for? Fascists? Liberals that wish to destroy the Soviet system and institute capitalism, thereby making the lives of the vast majority of people worse? Chapter XVII goes over this to some extent, but I of course do recommend reading the entire book.
I think Lumelore is starting of at anti-communism, and working her way from there. It leads to some weird stuff like this.