They’ve known this for at least a couple of months. But they had to keep Bernie off the ballot, and Biden was the only one who could do it.
They won’t run Biden in the general election. Someone else will take his place. It’s been part of the plan for months.
I think his VP pick, once revealed, will run the campaign, and Biden will become a mythical legend in the process, although he'll never be seen speaking in the present. Should they win (it would take unprecedented cheating, but this is the DNC), Biden will drop out and someone who never even had to campaign will take his place. HRC, anyone? (It won't be Sanders).