Wasn't GP talking more about valid but non-existent addressed? I'm guessing this is essentially the same as sending it to an address for which the key has been lost.

Yes, but that's pretty uncommon in practice. Why would anyone ask you to send money to a valid address that they don't own? It can't be a transcription error because the checksum would catch that

It's like sending gold over snailmail to a random address

And what happens if one sends gold over snailmail to a random address? You need to finish your analogy.