Even on a max effort, there's nowhere near the rounding that happened in your vid.
Anyway, obviously rounding your lower back is much worse. I'm just recommending you work on strengthening your upper back and shoulders otherwise as you go heavier, your upper back rounding is going to shift where your centre is and you'll find yourself trying to pull upwards too much. Note that your rounding was a lot more pronounced than in the video you're using as an example.
If you observe the top athletes doing sumo deadlift for example, you'll see how much leverage they get from positioning their upper body differently.
I'm telling you this because I had the same problem you did:
https://www.youtube.com/watch?v=2aa1VijSZrk (scroll towards the end for the farmer's walk practice).
My max deadlift was 795, at the time of the video it was around 700 tho (it's a bit easier with farmer's walk handles. The last set is with 380/hand). I went from having a rounded upper back at lower weights, to a rounded back in general as I went up in weights. It was starting to cause some annoying back pains and deadlift sessions always felt awful. I went to see a guy whose gym I trained that (Hugo Girard who was a top strongman) and got some coaching on deadlifting. I ended up doing a bunch of exercises like banded deadlift, really heavy upright rows, partial deadlifts, etc. Focused on pulling back (which allows you to drive your hips forward faster) rather than just pulling up and adjusted my form. Obviously there's going to be some rounding, but I kept it in check enough that I no longer had back pains and the weight went up another 95 lbs within a year.
This post was edited by CMBurns on Sep 7 2018 05:47am