The problem with those states is they were never true communism but deviated to support the ideals the founders wanted. In an actual Communist state you wouldn't have any classes, the political leaders wouldn't live in palaces or extravagant lifestyles and they most certainly wouldn't have dictators. That's the main problem with those communist states, they altered communism to fit their ideas, hence why we have Maoism, Stalinism, Leninism and Castroism. Each one is a different form of Communism and deviate from actual Marxist communism.
This is why Egalitarianism would be more close to Communism then the opposite. Losing private property wouldn't actually be a authoritarian ideal, rather setting up autocracy or oligarchies would be closer to authoritarianism whereas Egalitarianism forces you to only select true democracies which the ideals of True Communism would allow. This is another thing, Egalitarianism everyone has a voice, this is true with actual communism where the people have a direct voice in how the government operates. Egalitarianism also sets up a system where everyone in the state has equal rights which include citizen rights and human rights (though in Stellaris I suppose it's species rights) such as healthcare, food, housing, abolishing of currency and classes. Whereas with Authoritarianism there are still classes, most likely currency, land is controlled by the elite rather then the state and only the elite has a voice in government.
The main problem with Communism RIGHT NOW is we don't have the technology needed to actually accomplish it and people abused the concept (which can happen with any political ideology) but the whole point of Stellaris is it's the future, in fact like someone said a direct democracy would be the best way to show communism in the game with Subconscious Consensus being true communism.
Of course Egalitarianism isn't fully Communism but both share a lot of the same ideas and it's easier to role-play a true communist state as egalitarian then authoritarian.
Marx knew a lot about what he was talking about, it's just people abused his ideals so they can get more power.