I don't see this as Frodo's vision. This is the ringwraith's response to
a what if scenario. What if Frodo was not interrupted at the Crack of
Doom, and continued with his desire to be lord of the ring. Tolkien made
it clear in this same letter that it is his belief that if Gollum had not
retaken the ring, Frodo would have cast himself into the fire, with the ring,
when he realized he could not destroy the
ring.
If thou hearest the cry of the gull on the shore, thy heart shall then rest in the forest no more.
