It's about people who identify strongly with the Democratic party not fighting enough for the working class against the capitalist interests within. They're upset that so called "progressives" refused to see that the rich would never work with them. The only people who seriously stood against that shit were always viewed as outsiders. When the chips were down, people who weren't pariahs didn't push the party strongly enough. This has been confirmed more strongly with hindsight, but the leftists were always saying it, while the Democrats refused to believe it.

However, I don't think most politically educated Americans would understand wtf this user said. I'm just terminally "politics minded" compared to almost anyone. I actively try to understand how far left, far right, AND various establishment types view things. Regardless of how accurate their beliefs are, all people believe their own perspective to some degree. Even those that say things cynically can't help but internalize their lies.