Well the lower spine must be kept straight and stable. The upper back will round a little for any lifter lifting teh heavy weights. When you watch any of the top end powerlifters deadlift (like Andy Bolton, guy deadlifts 1000+), their upper back rounds a bit but the lumbar is kept stable.
Also, JDS was resting at standing position and dropping/bouncing the weight off the ground. Technically, this is not even a deadlift, but a "Romanian deadlift." A deadlift means that the weight is dead on the ground (hence the name) and that you use your strength to lift it to a standing position. After this, you drop or set the weight down. There is no negative movement in the exercise. Trying to lower a 600 lbs weight slowly and putting all that tension on your lumbar = herniated disc.
The Romananian deadlift variation is supposed to target the lower back more. However, with heavy weights this also leads to a slew of back problems. Like I said above, putting a lot of mechanical tension on your lower back = problems.
On second thought I guess 315 for a Romanian deadlift isn't terrible. It's hardly worth bragging about, but it isn't TERRIBLE.