This is an inaccurate claim by Sanders. Biden was the first President to walk a picket line with striking workers.
With union negotiations, he was pretty balanced. He did come down on the railroad strikes some after both sides got stuck, but they did get improvements beyond what management wanted to give. With the ports, though, he stuck with the workers and forced management to negotiate by refusing to override the strike.
Could the Democrats do more? Sure. But they're still recovering from the fever that took the party over with Clinton in '92. There are a lot of people who believe that win was a meaningful approval of the pro corporate but not racist Democratic Party platform, when in reality Clinton only won thanks to Perot.
I don't know that there was any magic messaging that Harris could've deployed this time around. I'm not sure there was any likely alternative even from a short post-Biden primary that could've done better than her.
Trump has everyone thinking he has some magic way to boost salaries and lower prices. And he railed against the elites more stridently and apparently people believed him.
On the other hand Google saw a lot of traffic with people asking if Biden dropped out so who knows what could have been done to avoid Trump 2.