Well since they had explained it already, I'll just stitch what I think is in my memory.
I don't know about absolute immortality, but yeah, they can't really die through physical attacks and if they were to die, Rimuru can just cast another revival spell and they'll come back to live because of some dimension pocket that acts like the cloud.
Hmm :-0 when I saw the main manga part I just thought that they could just revive not like the scene here where they literally put a knife through their heads immortal hahah that confused me X)
yea they are immortal now that army will be called the reborn! :D
SPOILER
The revived residence became immortal??
Also rereading from another perspective hurts still!