It doesn't need to specifically be Harris or Biden's policies to drag them down. There's very obviously a backlash against some progressive ideology going on, and the democratic party is clearly at least partly beholden to adherents of that ideology. That's why Harris can't give obvious and clear answers to (some) simple policy questions.
Yes, but the claim was they ran on that. Fixing the problem (if it is a problem) is a lot easier, and the necessary approach to fix it very different, if you ran on something and it backfired, compared with not running on it and still losing votes over it.
[edit] I also truly wondered if that’d been a significant part of their message, and I missed it—in the age of granular ad-targeting, who knows?