Rounding the upper back isn't something that's frowned upon... actually one of the most famour pullers, KK, deadlifts with a rounded upper back (while keeping his lower back flat aka... neutral).
Probably just his cue for flat back. If you're arched... then your back isn't in it's strongest position b/c it's curved and weight on a curved back isn't as strong as a flat back.
I concur with all of the comments and suggestions. In the end though the deadlift is the craziest of the three lifts people can do this shit so different and frighteningly and still pull it off. I always try to be exacting but know that trying small variations can yield a big return. I've had to concede a couple of form changes that I had a hard time watching but the results were worth while.