Mostly because of your explanation, I'll admit his wording was poor, but his message came across very clearly to me, what he meant is that the corpse has been alluded to be the corpse of Jesus Christ several times throughout this part, so it wouldn't really matter whether it has any importance to Johnny or not.
Johnny threw one of the Valentine's unloaded gun for him to pick up, to see if he didn't had a hidden gun on him. If Valentine was telling the truth, he won't need to have a hidden gun on him, so nothing would've happened.
But if he was lying, both guns would be attracted to each other(since they're guns from different dimensions), showing he had plans to betray Johnny(like happened in the story).