I’m not making any distinction. Socialism is socialism, capitalization or not, and the common definition is plainly Worker Ownership of the Means of Production. Whether done a la Market Socialism, where worker Co-ops form the economy, or democratic Socialism where there is liberal democracy that owns industry, or Marxism, Syndicalism, etc, this doesn’t change.
What is causing you to believe Socialism is authoritarian? If production is owned collectively, rather than by mini-dictators a la Capitalism, how is this more authoritarian?
As for 20th century Socialist countries controlled by Communist parties, such as the USSR or Maoist China, no leftist believes them to have been Communism, even themselves. They were Marxist-Leninist states attempting to build Communism via Socialism, in their own words. Some leftists call them red-fascist, or State Capitalist, but every leftist agrees that they had not achieved Communism.
Following the previous discussion, FLOSS is both Communist and Socialist. All Communism is Socialist, as all Communism is focused on Worker Ownership of the Means of Production, however not all Socialism is Communist.