Under communism, sovereign authority is attributable. If you ask the US president, they’ll say they have little power. If you ask senators, or congresspeople, or local representatives, the media, the bourgeoisie, neither do any of they wield power. Where authoritarianism occurs under capitalism, apparently no one is responsible for it. Under communism, it’s directly attributable to communists.

Both are often authoritarian, but the argument that communists are more authoritarian is simply an easier one to make.