>John didn't.
Both are correct for John's time had not come yet. But when Elijah did come for him, he did recognize him, the rest will have an excuse.
>how could he say that John the Baptist would not die when he was executed by Herod?
Because John had merged with Elijah. Elijah is the Spirit of Self Sacrifice and when that time comes to be the Son of Man, a Light to the world, most will make excuses as a form of not recognizing Elijah. Few find the door, and of those, only a remote few will ever walk through that door. For God's Kingdom alone.