If only 20% of the population can vote, and they own 60% of the population as non-voting slaves (the balance being women who were also excluded) you can say “slavery was the norm for the time, so was the exclusion of women from citizenship” and I’d agree with you: we can’t judge the societies of the ancient world by moral standards which were only invented in the last 300 years, or less. We are close to agreement, but to call it Socialism is a misuse of the term.
No wonder how wonderful the participatory democracy was for the small group of wealthy Athenian Slave-owners who enjoyed it, it wasn’t Socialism, it was, i think best described as democratic feudalism. Socialism is a political and economic theory of social organization which advocates that the means of production, distribution, and exchange should be owned or regulated by the community as a whole. As far as I know the Athenian City State did not confiscate all private businesses and run them as state collectives.
I have also heard of Socrates’ tragic end in a kangaroo court, which doesn’t actually shown Athenian society in the best light. Or maybe that’s where you think they are Socialist? Socialists are totally against freedom of speech, so purging philosophers who disagree with them is consistent with Socialism.