No Living Man

[The Saucepan Man]

Haven't we been here before?

Glorfindel did not say that no man could fell the Witch-king. He said that no man would do so.

Anyone could have killed the Witch-king, given the right circumstances. But only someone who wasn't a man would do so.

No other blade, not though mightier hands had wielded it, would have dealt that foe a wound so bitter, cleaving the undead flesh, breaking the spell that knit his unseen sinews to his will.

Like Formendacil, I highly doubt that this was the only blade capable of killing the Witch-king. Perhaps other blades wouldn't have dealt such a bitter blow in one stroke, but the likes of Glamdring, Anduril, Sting and many others would surely have been up to the task.