Communism is an economic system that falls apart in practice because of greed. An economic system that fails to account for greed is not good, even in theory.
If someone said "let's kill the Jews", you would probably slowly and carefully walk away from that person because they were nuts. An unreasonable proposition that you probably don't want to bother arguing against (because what kind of person would even say that?).
But it's only valid if you oversimplify a little. Starting with "first, forget everything you know about human nature" goes too far.
I don't think of "works in theory" as shorthand for "doesn't have internal contradictions" or "sounds reasonable if you forget everything you know about human nature."
I can see how one might find communism interesting from the perspective of 19th century. I cannot believe anyone still does in the 21st.
No we don't want communism (at least I don't), but for someone to say it is illogical is completely incorrect.
Fascism is quite similar, though I'm not sure one could say communism is worse than fascism, given that the latter was only tried a couple of times and was put out relatively quickly, while the impact of the former happened over a much longer period of time and under much larger populations.