If a guy focuses 99% of a fight on striking and trying to finish a fight on his feet, it is almost guaranteed that his bjj game is going to look weak because that isn't where his focus is.
Silva gets put on his back, he doesn't really try much of anything except to land some strikes and try and get back to his feet. Because of this, it looks like he doesn't have a good jitz game.
Now, I'm not saying that his jitz is therefore good or anything. Just that you can't assume that his ground game is poor simply because he doesn't prefer to use it.
Hell, anyone that has a blackbelt from Nog has a good ground game IMO.