Zercher deads are a very cool lift, but I think the added stress on the hamstrings instead of the back is the main reason why the back is not injured while rounding even though it would be bad if it was a conventional deadlift. Here's some info on it I came across:
For the Zercher deadlift the bar begins much lower - on the ground if possible, or at the very least on the lowest pins of a rack. Taking a wide stance, the lifterís arms are hooked beneath the bar in the same way as in the squat. The hips are lowered, and then the lifter raises the back and straightens the legs simultaneously. As the bodyís centre of mass has been shifted forward by holding the bar in this unusual way, much more stress is placed on the hamstrings than in a traditional deadlift; which focusses on the spinal erectors.
Once my deadlift starts plateuing I'm going to start z-deading every week because I actually know someone who gained like 40 pounds on their deadlift just from focusing on z-deads for a while