This is the unfortunate side-effect when social and political signifiers become snarl words. I haven't done a science on it, but it seems like most people who use "left" sans detail and don't consider themselves leftist bundle DNC, DSA, and all the other center-left factions into one and imagine them being led by whoever the current DNC leader or president is.

I double checked /u/givemeethekeys' post and it's hard to pin down a definition. They mostly seem to want to accuse Biden of being a Republican while also insinuating he's a leftist, so their whole analysis is deeply confused (or confusing as presented).