The thing is that according to liberal ideas, the economy IS doing great.
Anyone to the left of Joe Biden recognizes that it isn’t, but liberals are the majority of the democratic party. To them the “traditional” economic markers are the most important things to track, and those numbers all look good.
“Liberal” economic ideas have nothing to do with the living conditions of the average person, they have everything to do with the capitalist class being happy and “opportunity” being available to the working class. That’s why the economy in the 1910s could be described as healthy even though people were literally forced to live in tenement houses and were being locked into factories. That’s also why many liberal economists say that it’s possible for unemployment to get too low, because apparently that’s considered bad.
The question Joe Biden is answering when he says the economy is doing well is “are the capitalists happy and continuing to expand capitalism to extract as much wealth as possible from the working class,” and right now that answer is clearly yes.