Not sure I understand the complaint about undermining the democrat's messaging now that the election is over (Sanders was very locked in with the Democrat's during the campaign). Identifying a reason for a loss and using that as a rally point for future organizing is appropriate in the wake of a defeat, and certainly more productive than the "Harris ran a perfect campaign" solipsism that is abundant on TV and online.
I agree Bernie overstates the case against the Dems (particularly under Biden), and the GOP's record for working people speaks for itself. But it's also true that Harris retreated from an economic populist message early on in her campaign. Per an Atlantic piece today that was on the advice of her brother-in-law, Uber general counsel Tony West (who, incidentally, was also the architect of the recent ballot initiative in California that compelled gig workers to be classified as independent contractors and not employees). That's when you start seeing Mark Cuban, a literal billionaire, acting as a prominent campaign surrogate. Her being cagey about the fate of Lina Khan in a Harris admin was also not helpful on that front, nor was her public disavowal of single payer, nor was the overall campaign strategy of courting suburban professionals and disaffected Republicans. IMO the overall approach prevented her and Walz from effectively conveying the economic message.
To the extent the critique is: "Democrats need to be more consistent with signaling their support for working people and then delivering the goods if they want to win back the voters they've lost," I think it's correct. I am skeptical that shift is possible given the institutional commitments of the Democratic Party.