 He's planning to write more about this in the future. My strong sense is people who want Biden out would be perfectly happy with Harris, even if they prefer some sort of open nomination process, whereas people defending Biden are more indifferent between Biden and Harris. In technical terms, Harris is probably the choice that would emerge from a negotiation and a game theory equilibrium, not necessarily the party's best option to defeat Trump, but the one minimizes the loss function for respective party stakeholders.