All the Dem candidates will be there and Fake News will compare it to GOP not supporting POTUS.
That's a good point. In 2016 Cruz, Kasich, Bush and maybe other former primary candidates did not support Trump's nomination and made a point of letting us know about it. Two of those three still can not support him.
It is not Fake News to say that the GOP is not supporting POTUS. Practically the entire Republican establishment is arrayed against him, including his entire former cabinet and most of the WH staff. The Dems are unquestionably more united than the GOP. It's smart for Biden to make a show of it.