I think the clause where cabin crew can elect to evacuate in a catastrophic failure is more for when the flight deck is inaccessible, or the pilots are not conscious or there is an imminent threat to life onboard.
Imagine you elect to evacuate, all doors are opened and passengers evacuate into the live engine?
In this case, the pilots would have made an assessment, backed up with outside assessment from fire crews etc., and made a decision based on that.
With one wing down, you have slides on one side at a shallow angle and on the other side at a steeper than normal angle, which would add to the chance of injury.