The authoritarian left are communists, which is where the state owns and operates everything. The libertarian left is a decentralized society with the people owning and operating everything. Those are the only two positions that can accurately be defined as the far left. Bernie is neither of those things. Capitalism, by definition, is right wing. Bernie isn’t some radical who wants to overthrow capitalism. He would leave the current structuring of society in place. He just wants the working class to be treated well. That’s what makes him a centrist.
The reason why he gets called far left is because Americans have been bombarded with decades of MSM propaganda to never, ever question capitalism. The entire debate exists within the capitalist structure. Meaning, the center gets called far left, center right gets called the center and the far right gets called the right. Basically, modern Democrats are moderate Republicans who are okay with abortion. During the Cold War, America was taught to fear a communist government so much that they ended up with a fascist one instead.