I actually find this a very plausible conspiracy theory, based on how events occurred. Clinton was in a heated primary with Obama, and faltering. She graciously stepped back without fuss, was made Secretary of State, and was laser-focused by the r’s for 8 years trying to pin a scandal on her. When Obama left, Biden declined to run. All of this suggests a deal made for after the Obama presidency, and the r’s hearing about it (notice she was the single target after him, they never attacked Biden).
I would posit that some deal maker traded her backing away cleanly for promised delegates and a clear shot after Obama. I don’t know the background structure of the d’s party, so I have no idea who it would be.