To put it in terms similar to your statement, I don't think his actual condition is as important as the backbiting talk. Sufficient backbiting will tank his chances no matter what his condition. If the backbiting stops, he has a chance even if his condition isn't great. We know he can hold it together for some periods of time, and that might be enough. As a supporter, I can't change his condition. But I can try to stop the backbiting.

They're not unrelated.

The "backbiting" stems from his condition. A major reason that party leaders and elected Democrats are so upset is because they believe the campaign isn't being candid with them. That they're trying to rely on the fact that Biden can "hold it together for some period of time" to create the impression that Biden's in better shape than he is.

They don't think that's fair to them. They're being asked to go to their constituents and rally them for Biden. Because of the debate performance, they're now on notice (in the eyes of their constituents) that Biden might have some problems. So now, when these electeds and party leaders vouch for Biden to their constituents (an important part of campaigning) it puts their necks on the line. Because if Biden goes out and sh!ts the bed in the September debate, or during some campaign stop, it's going to destroy their campaigns and election prospects and credibility with their voters.

So they don't want Biden and his inner circle to hide his condition. That's no longer a feasible alternative for anyone other than Biden. They want them to be candid about what it is. And they're not going to risk a huge swath of the party without knowing what that risk is.