Today IGN spoke to Jason Momoa about his new show The Red Road and asked him about the rumor of him being in Batman vs Superman. He clearly denies these rumors as nothing more then such. Rumors.
Should we believe this? No not really. Because Variety is the source that broke the news that Momoa is up for a role in Batman vs Superman. The same thing happened when Variety broke Paul Rudd was up for Ant-Man and the actor denied it or Benedict Cumberbtach isn't Khan. They are legally obligated to deny everything until its finalized.
He may also have been up for something at one point and has since backed out just like when he was up for Drax the Destroyer in Marvels Guardians of the Galaxy last year. Maybe Dwayne Johnson is actually taking over the role Jason Momoa possibly passed on?
:: Update :: -
IGN has updated their article with quotes from Momoa
"Maybe I told someone that it would be awesome and that's how it started. But it's just a rumor. I mean you audition for things..."
Batman vs Superman hits Summer 2015