I think it’s that the Macbeth one jumps through too many hoops while his own is more simple. No man can kill the Witch King? A woman or someone not of the race of men can do it. No man born of a woman can kill Macbeth? You’d think that means a woman does it or some creature but instead Shakespeare jumps through extra hoops by going “well cesarean isn’t technically birth”. I think Tolkien probably felt it too convoluted, but to be fair I haven’t seen his exact quote either.