Easy bigguy, just giving my opinion on an opinionated board. My best for reps in all honesty is less than what he's doing for reps. Using a strength coach from UNLV I managed to pound 405 for reps and then quickly moved away from deads for the summer. The only reason I mention about form is because on video I did the same type of technique as him and I made tremendous progress by switching my form so I will agree everyone is different and he could possibly benefit as well. Only he can make that call. For me my legs where the strong point, not my back. Just my .02's...its all good...
Don't forget that he was doing stiff legged deadlifts. If he would have wanted to pull a maximal single rep he would have bent his legs more.
There are tons of variations and exercises you can do to build your deadlift to strengthen every part of the chain. For example I do good mornings seated, standing, straight back, sometimes slightly round back, wide stance, short range, full range, concentric etc. Same with deads; straight leg, bent leg, arched back, rounded back, deficit deads, deficit with snatch grip, rack pulls, speed pulls and on and on.
Looking at Chris' strength I'm sure he has put in a lot of time and thought into his training so I doubt he is rounding his back out of ignorance. See what I'm saying?