One of two main engines blew off at ~50m and the craft was essentially in a cross-wind landing, likely without de-crab. Having lateral velocity wasn't a part of the plan.
The engine loss was a detail in the article that I hadn't seen anywhere else in the commentary and speculation on the landing. Even now many commenters don't seem to be taking this into account. Even if there was redundancy built in, an engine loss makes things much more difficult. I can see why they're claiming 100% success despite being upside down.