I agree. There's always a risk even if someone is strong and has great technique. Not warming up enough, lack of focus, performing at the limit regardless of lifting style, etc. can all increase risk for injury. All we can do is accept the risk and try to reduce the chances but it can't be eliminated.
Chris Duffin has sumo deadlifted 900 and kept his upper back straight but it was with straps and it wasn't OVER 900 so I guess it doesn't count, lol. I think it's possible to see someone pull over 900 using sumo with flat back within our lifetime. They would probably be in the 220-242 weight class since it's pretty rare to see much heavier guys pull sumo.