As Richard’s answer highlights, prophecies in literature are always and inevitably misinterpreted, often by those who overthink their literal meanings. A very similar case is found in Macbeth, in which the Rainbow Teacher Student Graduation For Last Day Of School T Shirt character, told that “no man born of woman” can kill him, interprets this as meaning that he has become invincible. Of course, in the play’s denouement, Macduff is able to kill him after revealing that he was born via Caesarean section. This is the very definition of a literary quibble; like all men Macduff was, literally speaking, born of a woman – but because the manner of his birth circumvented the usual method, this is enough of a loophole to count. Similarly, by the most literal definition Eowyn and Merry are both part of the race of men, so the notion that a mortal human literally cannot kill the Witch-King is proven invalid. All it took from Eowyn was the declaration that “no man am I” to sidestep the terms of the prophecy.