You're gonna hate me, but I respect him for making that comparison, because most democrats don't have the balls to. He's right, there is a rising dissatisfaction and populism on the left, who aren't feeling the strong economy, and it mirrors what happened on the right. It's less bad than the Trump-Republican thing (which is a national crisis) but the idea of blowing up the system with tantrum votes is gaining momentum and pushing people toward the extremes. And Bernie, for all his good, is an extreme within the party. He could have said it with the caveat that Bernie > Trump, but he's also competing and looking to clear some space for people who aren't ready to go full Sanders, so I don't blame him.
And one of the few policy issues Buttigieg has spoken about is raising taxes on wealth, which is meant to address this problem.