From an unusually good source:

Obama’s tweet supporting Joe Biden was disingenuous. In private, Obama is telling people Biden can’t win, and he is therefore in favor of an open convention. Obama will not say whom he supports, nor as of yesterday afternoon had he met personally with Biden to deliver the message.

Relations between the Obamas and the Bidens have never been warm. At times they’ve been hostile. But recently they’ve deteriorated further, mostly due to Jill Biden. In the hours and days after the debate, she kept her husband cloistered away from anyone who might convince him to drop out. Jill Biden is the driving force behind her husband’s reelection campaign, just as she was in 2020, when other members of the family (including Biden’s sister Val) considered him too impaired to run.

The next generation of potential Democratic candidates understands all this as an opportunity and they’re circling, particularly Gretchen Whitmer, who is promoting herself aggressively.