Yeah, my take on why Grate isn't immortal is kind of personal (in that I'm not sure I've ever heard someone else articulate it this way)
It's similar to why perfect clones are not me, and I usually describe it with the idea of 'stream' or 'chain' of consciousness. The idea is that, in order for me to survive, there has to be an unbroken chain of consciousness back to this point. (putting aside discussions as to whether sleep or being put under anesthesia breaks this, which are interesting philosophical discussions in their own right) In this framework, a clone is either the creation of a branch in this chain, and the nanosecond after you've created the clone it is no longer me, or a clone is the creation of an entirely different chain that happens to be completely identical up to the point of it's creation. Either way, if you kill me, and end the original chain, I am done. I am not that clone. My experience will cease. And while, to the outside world, there will indeed be absolutely no difference, and the clone will be just as convinced as I was that it is me, I will be just as dead. (This is, incidentally, why I will never use the most practical variation on teleportation if it ever exists)
To me, this is what Grate is. He is the creation of a clone at the moment of his demise. The true Grate, the original chain, ended with that rock. All we have now is copies, and while we cannot tell the difference, (and for the purposes of the game, the distinction is completely irrelevant) it is important to mark it. This is not immortality, as I would desire it or even accept it. But it is a form of something close.