The "PFF stuff" is the gold standard when it comes to player performance appraisal. I'm not saying that it should be looked to as the ultimate authority, but they're the best at what they do, and in most cases, their analysis passes the eye test (see Jamal Adams and Quandre Diggs).
Besides that, just what are you basing your statement on that Olu did "really fine"? Outside of special teams, he only played in 4 games.
Olu and our first pick in this draft, Byron Murphy, are both relatively the same size, ie 6'1" 300 lbs or thereabouts. Murphy ran the 40 in 4.87 seconds and had a 33" vertical jump. Olu ran the 40 in 5.38 and had a 29" vertical jump. That probably says more about Murphy than it does Olu, but it's a demonstration of what kind of challenges Olu faces. He's not a road grader, tends to get manhandled in the run game because he's not very athletic.
IMO Olu is still very much an unknown quantity, and what little evidence we have isn't good. That 5th round presumptive starter tag is a misnomer.