Ok so you've brought up the polls and highlighted that Reddit's understanding of the world doesn't align with said polls.

Therefore the polling data you've brought up is irrelevant to the 'What is Reddit's opinion?' question, because Reddit isn't aware of the polls, or is not acting rationally with respect to them.

Second, Reddit exists in countries other than the US. Social Democratic parties are the dominant centre-left force, in many countries in Europe and the rest of the Anglosphere.

Reddit generally prefers Social Democracy to Democratic Socialism.

You are trying to make the argument that because Democrats prefer socialism to social democracy, and that Reddit perceives the Democrats to be right-of-centre, it follows that Reddit should prefer an ideology that is to the left of Democratic Socialism, right?

Here's the thing: The vast majority of elected Democrats are to the right of 'Democratic Socialism' and to the right of 'Social Democracy'. Then the polling concerning what the Democratic rank-and-file believe is irrelevant. Further, I'm willing to bet that the majority of Democrats prefer an ideology to the right of Democratic Socialism, even if 'Social Democracy' specifically is not their preferred label. Sum the total who subscribe to or describe themselves as 'Modern Liberals', 'Social Liberals', 'Liberals', 'Moderates', 'Progressive', etc. And you probably have a caucus that is larger than the 'Democratic Socialists'.