‘Negotiated’ is a fairly liberal characterization.
Basically the two campaigns got to nominate people to the Unity Reform Cmte., which wrote the rules for this cycle, something like a third were DNC representatives, a bit more than a third from the winning campaign (Clinton) and a bit less than a third from Sanders’s folks. He wanted to do away with supes entirely, but they didn’t agree so they arrived at the 2nd ballot solution, which none were really happy about. They however also fought to keep the caucuses, which they succeeded, and tried to get all primaries become open, which they didn’t.
Saying Sanders wanted to keep supes is wrong, but then again the reasoning that since they are still there, trying to make a play for them is ok since that was something Sanders himself did is valid.
He was a part of private negotiations with Hilary which resulted in the current rules. They wouldn’t be in place today had both parties not come to an agreement in the end.