My definition of manipulating the vote would be changing the results.
They didn't do that. Clinton got more primary votes than Sanders. Biden got.more primary votes than Sanders.
And the reason they didn't like Bernie makes some sense. Bernie wasn't a Democrat. He was an independent who decided to run on the Democratic ticket because he knew that running third party would split the vote. But the Dems didn't like the outsider trying to take over the party. They saw it as an attempt at a hostile takeover.