Whether Harris can actually deliver on those promises or not, has almost no bearing on the actual election - she has to say it, and she has to say it credibly, and to do that CREDIBLY, she's going to have to admit that Biden blew it.
This party is not in the "admitting Biden blew it" business. They're in the "okay but let's talk about how we got gas prices down 5 cents last July" business. And nobody is hearing that shit this time because this is existential terror moments.