I don't understand why a engine out scenario would be slower? Descents are performed with engines idle. An average of 3,500 fpm seems pretty consistent. If they were using the speed brake as well it might have been more which might suggest they had airframe damage.
Also depressurisation could be a scenario, hypoxia may have prevented them from making fair assessment of the MSA.
All I can say is that I feel very sorry for the people in that situation.