There is an alternative take, which is that the Dems concentrated their efforts in certain metro areas. I'm sure they pushed much harder in competitive states than in places like Chicago, New York and San Francisco.
I also think the focus on central cities leaves out a big part of the story. Dem margins grew by very large amounts in the suburban counties around Atlanta and Philly. Also Maricopa County and Pima county in Arizona. And on a smaller scale Chatham County (Savannah). Allegheny County (Pittsburgh) padded the Dem margins by around 40,000 votes.
If you look at population trends, the growth is still very much in those suburban counties, and away from rural counties and central cities. To me that is where the main difference between 2020 and 2016 lies.