OK, my bad. The expressed antithesis of Popper's open society was dictatorship, not communism. There is little doubt that Popper saw communism (and even marxism) as expressions of that antithesis (i.e. by necessity leading to dictatorship), based on his own hard-earned experiences (and by no means questioned by Marxist-Leninists). But agree, capitalism is by convention the counterpart to communism in that habitually dichotomous concept of possible economic models, but does not guarantee freedom from dictatorship as you correctly point out.