Is it possible that it wasn't a miacle which revived Kirito and made him able to kill Heathcliff.
I think, Klein revived him with the revival-item which Kirito earned from the Christmas Boss on floor 50. After Kirito realises that he can't revive Sachi, because it only works if somebody use the item on somebody who died 10 seconds ago, he gave the Item to Klein.
And with this item klein revived Kirito and then he killed Heathcliff and beat the game.
Original Wikia Entry:
"Thanks to a miracle in the game allowing Kirito to launch one final attack after his HP ran out, Kirito manages to defeat Heathcliff and free the trapped players from the cursed death game."
Sources:
http://swordartonline.wikia.com/wiki/Heathcliff