Bernie is just a better politician than most. He knows identity politics are a losing effort. I'm thinking the left is clinging to it because it allows them to appease voters without having any tangible plan of action and without explicitly promising anything to the electorate.
The problem is that the people attacked by identity politics (men, women who believe in traditional gender roles, and people who are opposed to government authoritarianism) are starting to talk and find common ground where none existed before. This is dangerous and could lead to the left being represented only by gay and effeminate men, the LGBT community, and pc moral authoritarians. Groups of people who were and will be powerless again if they're not able to wrap themselves up in the cloak of "marginalized people"