It seems to me that the problem stems from you thinking communism necessitates authoritarianism. Communism is an economic system. You say you consider yourself a democratic socialist, while there is obviously a bit more nuance, in the absolute basics, that is saying the economic system you believe in is socialism, and the system of governance you believe is a democracy. Someone saying they are a communist would be the same situation as you saying you are a socialist, it's true but it doesn't state your full political beliefs. I obviously don't speak for the person you are responding to, nor can I assume that they have the same belief about this subject as I do, this is simply my interpretation of the disagreement, and my stance on it.
Additionally, I would like to respond to your earlier mention of asking a communist to give you an example of a communist country that worked out in the end. The reason many people respond negatively to this is because of the history of communism, especially in relation to the US which is where much of Lemmy is from. The US has a history of intentionally destabilizing communist(and socialist) countries, as communism is inherently a threat to a country so heavily built on massive corporations. Because the US and other countries make such a point of preventing communism from succeeding, it can be frustrating when a lack of successful large scale communism is used as proof that communism can't work. Additionally, because this same argument is used so often, it can really begin to grate on someone's nerves after being asked it over and over again.
I have tried my very best to not make any assumptions about you, other than the political ideology you stated you had, but if I accidentally did, please tell me. I do not wish to offend you, and rather just want to provide my input on what you have said.