If someone is good off their back that means he ca either submit from his back, or be able to reverse the position. I don't really know what makes you think he able to do that.
He has one submission in his career against a pure wrestler in a time when BJJ skills and submission defence weren't where they are at now.
You will have to go way back to pride days to even see him reverse someone from the bottom. (I don't even remember it happening I'm just assuming he did)
But people still want to make it seem that if Shogun is on his back Shogun has him right where he wants him
They also mention that Shogun will be throwing a number of kick at Jones' lead leg. Despite the fact that Shogun clearly wants to keep the fight off the ground.