Bernie Sanders (since you're presumably talking about the US) doesn't even support open immigration or H-1Bs, let alone open borders.
I think 'liberal progressives' that are mainly concerned with identity representation, not working class power, are rhetorically bundled as 'far-left'. To people who buy that, Bernie Sanders (a Leftist) and Kamala Harris (a Third Way liberal progressive) are both far-left. It's incoherent but rhetorically potent.