It's pretty simple - communism is "real communism" when it works, and it's not when it doesn't. It's similar to how it's not "real capitalism" when things go wrong in a capitalist system.
Cuba isn't doing too badly even though it's authoritarian. It's no soviet union, DDR or North Korea. The biggest problems they have are due to US sanctions.
But I think extremes never work. Neither strict communism nor all-out capitalism.
I’ve been wondering the same but maybe we only have these examples because the only successful attempts to forcefully take power away from the capitalists has been by authoritarians.