They don't know they shall die, but he goes even further saying, they don't know anything. Why? Simply because they are dead.
Again, you're assuming the very point that you have yet to prove - ie dead = non-existent. If the writer is simply saying that the non-existent know nothing, he could be accused of stating the b£**ding obvious. Why would knowledge of any description even enter into consideration if there is nothing there to contain it?
The statement only has meaning if there was something still there with the potential to have knowledge. Put another way, the mere absence of knowledge does not prove non-existence. Or in your parlance - soul death/extinction.