If there is anything to compare, its possibly with the unseen side of things. Elves can walk both in the seen and the unseen world. Mabye thats what is implied here: that while a mighty enough elf could destroy the physical rainment of a Nazgul, the undead spirit could never be destroyed while Sauron existed. Hence the "Nazgul can never be defeated" quote. An elf could clearly see that.
The unhoused Nazgul spirit would simply recloak itself (in time) in another physical shell...?..?