At this point in the movie no one had any idea that the torpedo tube had made a soft landing on Genesis and that Spock's body had been rejuvenated.
The crew of the Grissom did know of the soft landing. Starfleet did know of the soft landing, because Esteban had insisted on reporting it ASAP. And we can assume Kirk kept a close eye on what Starfleet knew, because it's explicit that he had immediate plans of returning to Genesis at the earliest convenience quite regardless of any katra business. He had said as much at the end of ST2 already.
Kirk didn't know that Spock's body was alive again, no. Nobody except David and Saavik knew that, since Esteban's last message had been jammed by the Klingons. But apparently, even getting the body back was something that Kirk had always considered worthwhile: he knew that Genesis was "life from death", and he stated this as the reason for his intent to return to that planet.
Perhaps Sarek didn't mean that (although we do know Vulcans have interstellar-range telepathic abilities, and Sarek might have known a thing or two about what was going on at Genesis), but Kirk would have misinterpreted Sarek due to his own expectations...
Timo Saloniemi