I think what they mean is that campaigning is a lot more about the candidate’s personality and determination, which Biden lacks. Whereas “presidenting” can just be about signing some paper and making public speeches, while the rest of the cabinet does all the hard work.

In a way this is disregarding all the volunteering, marketing, financing, that happens during the campaign, but they’re not wrong in that in people’s minds, not looking fit for the job has a lot more negative impact when you’re trying to get the job, than when you’re already appointed.

Also it takes a lot of energy to run a campaign, you have to face all the criticism and you’re the one debating and trying to convince people. When you’re president you can just send a random scapegoat (Macron has done it numerous times when personally addressing issues would have hurt his public image, so he sent a minister instead)