For every Felder, there was a poirier, ferguson, giagos etc etc. I'm not saying that oliveira on top would be instant death, but it could certainly contribute to him winning a round and altering the trajectory of the fight. The only time we've really seen islam on his back in the ufc was when he sat in a guillotine for like 90 seconds against wade, and again against wade while islam sold out on a triangle attempt while getting punched in the face. He lost that round, and chris wade is no oliveira. Even back before the ufc, he was very obviously nowhere near as comfortable off his back as he is on top.
The fact that DC mentions oliveira working his BJJ from top position as the worst case scenario only corroborates this. It's clearly not a threat to just brush off.