It's definitely not strictly a leftist anime, but it speaks to the same people on the left who think Death Note is a good anime. Death Note is about a sociopathic murderer and in no universe should anyone think Light is a good guy. Code Geass though is a much more realistic interpretation of the same story. A guy with basically godlike powers who is trying to use them to genuinely better the world....and all of the problems that causes. Rather than just murder murderers like Light does, Lelouch actually uses his powers to take down an imperalist monarchy and free the oppressed. Or, well, he tries to. Often to horrible results. Which... to me at least is more true to life, and it makes it a compelling story. It's a much more left-aligned goal, and since the two anime are often compared, I totally understand why people think Code Geass is a leftist anime.

I personally love Code Geass, flaws and all. It's a great anti imperialist story and watching giant robots and mind control tear down imperialism is just fun. Is it a leftist anime? Not really. Is it a fun anime for leftists who hate imperialism? I think so.