He's not saying they cheated Bernie, just that they were more inclined to defeat him than they are Trump.
A crowded field has advantages and disadvantages based on the candidate. Bernie's biggest problem has been a lack of attention or credit, low information voters which make up the majority don't take him seriously because the rare times he's mentioned is usually in a negative or even false light.
Bernie's strength is his tenacious focus on policy and his ability to communicate how that policy affects the average American citizen. Joe's weakness has been a lack of energy campaigning and weak debates where he can only ride the coattails of the former president without offering anything new.
So they pack the field up until Super Tuesday where the entire field drops and supports Joe who had performed terribly until that point. This gave Joe a huge surge of momentum with many low information voters flipping to him at the last minute. From there they at least at enough for a brokered convention and could continue the narrative that Joe is electable and Bernie isn't, which is a big deal for the people who just want Trump out of office.
As for motive, the DNC supports their establishment candidates and have burned Bernie in the past. If he won the nomination, Bernie would have cleaned house in the DNC and restructured it to be more progressive with like minded individuals. So if the DNC could get their guy in, they may lose to Trump but the status quo and their places of power remain and they stand a much greater chance of winning the next election without a real successor to Trump in the Republican party.