Well if you’re only polling Democrats, the poll isn’t giving you any real understanding. The actual socialists have been methodically and intentionally pushed out of the party. And young folks have clearly heard the message, as they have broadly rejected the Democrats in return.
There’s a reason the Democrats are polling even lower than Trump, and that’s precisely because of how many people have embraced socialism. Trying to understand that only within the context of those still with the party is pointless.