"King Cohn" details the connections and dynamic, as does "Handsome Johnny". The mafia dynamics of the Cohns and Columbia Pictures are well documented and not disputed within Hollywood:
Cohn also had a number of ties to organized crime. He had a long-standing friendship with Chicago mobster John Roselli, and New Jersey mob boss Abner Zwillman was the source of the loan that allowed Cohn to buy out his partner Brandt. Cohn's brash, loud, intimidating style has become Hollywood legend and was reportedly portrayed in various movies. The characters played by Broderick Crawford in All the King's Men (1949) and Born Yesterday (1950), both Columbia pictures, are allegedly based on Cohn, as is Jack Woltz, a movie mogul who appears in The Godfather (1972) as well as Rod Steiger in The Big Knife.
https://en.wikipedia.org/wiki/Harry_Cohn
Most will remember the horse head scene from The Godfather -- that character is the "Jack Woltz" referenced by the Wikipedia entry. Obviously it is not a secret. What Cohn did to Sammy Davis Jr. was horrible and well known.