The OP's claim that communism implies authoritarian government is likely based on the concept of the "dictatorship of the proletariat" in Marxism, and this is a quite common view [0]:

"In The Road to Serfdom (1944), the Austrian School economist Friedrich Hayek wrote that the dictatorship of the proletariat likely would destroy personal freedom as completely as does an autocracy. The European Commission of Human Rights found pursuing the dictatorship of the proletariat incompatible with the European Convention on Human Rights in Communist Party of Germany v. the Federal Republic of Germany (1957)."

Of course, Marxism is just one version of communism among several.

[0] https://en.wikipedia.org/wiki/Dictatorship_of_the_proletaria...