My understanding of this chapter: she DID kill him. When she stabbed him in his throat he spit out a drop of blood that landed on Unohana's cheek, that drop was still there when he regained consciousnesses. Not entirely sure if the "instant healing"/"denial of death" was Unohana's ability or Kenpachi's... but it was not an illusion.
I'm more excited about the fight we're gonna see in the flashback than the fight we're seeing in the present to be honest. Could be a fight between a man that couldn't die and a woman that could kill anything...