Deadlift for reps is sometimes one of the strongman events, I think that is why he trains that. If its the video I think it was, there was a "gym competition" last year on long island where the money was for as many reps with 405 as possible. He walked in and took the cash.
So I guess it depends what your goals are. Do you want to get better at deadlifting a light weight for max reps or do you want to get better at pulling max weight. As this is the powerlifting forum, I would have to think it's the latter.
I suppose high reps would work to increase your 1 rep max in the deadlift, but I can think of better ways to improve it than doing 20 reps with a light weight.
Oh, and I have done max reps deadlifts in strongman competitions with relatively light weight. To me its a test of conditioning, not of strength. I suppose its pretty much the same for high rep squats.