The Rock never specifically stated who he was going to play before he announced Black Adam. He hinted at John Stewart a couple of times in twitter hashtags or interviews, then around comic con he announced he was going to be Black Adam. Tyrese doesn't hint at anything, he blatantly says he hopes to play John and posts fan pics of himself in the role, quoting the GL oath. If there is some truth to it, that's not how WB usually conducts their business. Jason Momoa didn't say a word about his casting before it was announced, despite that they knew we were all aware of it. Makes no sense to me why they would go 180 here.
Edit: You know, the more I think about this, the more I'm convinced it's actually true. He just post another pic with "20|20 I am fully committed to suit up for this adventure". Now, if he was lying about this whole thing, why would he be aware of when the movie comes out? He wouldn't care about details like that if he wasn't associated in any way with it. So for now I'm inclined to believe this story. Very disappointing to say the least, I hoped they would give Hal another chance and better portrayal this time especially considering they seemed to be going with the New 52 Justice League line-up, but they probably thought they should get away from the 2011 movie as much as possible and from a recognition standpoint, it does makes sense to go with John Stewart. I'm not pleased neither by this, nor the casting choice but of course I will give it a chance. I can only hope they make a better movie this time.