There’s not going to be any “proof” there. It’s a pretty ridiculous claim for a single source to be making considering how huge it is, and how many secret/behind-the-scenes rooms this story takes place in. That one source would need to be in almost all those rooms firsthand for any of this to make sense, and making sense isn’t a huge concern considering the story opens with the notion this whole thing was an Iger-led game of 4D chess whose ultimate goal was preventing WB from competing with Marvel Studios.