"Like I told Barack, if I reach something where there's a fundamental disagreement we have based on a moral principle, I'll develop some disease and say I have to resign."
In the interview Biden says that when he was VP under Obama, that if he had a moral disagreement with Obama, he would have developed a disease and resigned.
If he has one now with Kamala Harris, he as president, would resign.
A strict interpretation of that interview is more than a bit bizarre should such a disagreement happen. But It does sound like he is kidding.
And the point of the interview was that he and Harris would not have major moral dispute.
Regardless, this will play into allegations that Harris will call the shots and that Biden will not last four years.