I don't think that it means that if Torvalds gets shot tomorrow that the Linux kernel will die. I think that what it means is that there are parts that only he is knowledgeable or invested in enough work on in order to fix certain bugs. That is, if he were to die tomorrow, other would be able to take his place, technically, but they would have to spend a while reverse engineering the code and might even make significant changes to suit there tastes.

This happens all the time with commercial products.