The katra is not canon therefore Spock and Kirk do not believe in it. As far as we know from the movie it is only a superstitious belief about the after life. The Vulcans only 'think' they are communing with Surak. The young Spock was Spock. The telepathic energy he gave to McCoy awoke his memories is all. He still had them. He was just dead. He regenerated but his core was still there. He was reborn.