it's a pretty big move competing in the opener... you couldn't see him doing the ironman at this point in his career. I guess he figures he wins it, and he's a frontrunner for the O, and if he loses, he can just say he wasn't trying to peak yet. It'll make the show big news, no matter what. Some smaller name knocks off Dexter, it's huge.
In my opinion, there may be more pressure on him to win the Flex Pro, than to win the Arnold.