Can anyone think of any other reason why Democrats lost [the working-class vote] in 2024?

I think that's a different question than asking whether the Democrats need more policy plans. The election loss in 2024 - especially the Presidential race - is overdetermined. Tons of possible causes that are large enough to explain the loss.

The question in the article in the OP is, I think, a little narrower: why didn't the Democrats' actual policies help more? That's not unique to those authors - there seems to be a growing consensus that Bidenomics just wasn't popular, a recognition that extends even to groups that were big fans of the substance:

prospect.org - Apr magazine retrospective on bidenomics

The OP article basically says that you can't win elections without strong parties, and that the bones of the party aren't their policy plans. They're structural, institutional, coalition-building organizations - they're a thing, not an idea. You can't rebuild parties by having better ideas than "the other side," because it's not the ideas that get people to support a party. A party is a team, a group of aligned people...not just, or even primarily, a platform.