^ The only trouble is that Kirk makes a point of saying "Captain Sulu", with an emphasis on 'captain' as if Bones doesn't know about Sulu's promotion. Or maybe Bones does know, but has simply forgotten. Old age does that to a person.
I'd say it was Kirk's way of reminding McCoy that Sulu has other responsibilities now. I'm sure McCoy was aware of Sulu rank and posting.
That's how I interpreted it.
McCoy looks around, notices that the old gang is here, except for Sulu, who is conspicuously missing.
Kirk reminds McCoy that Sulu's new job means that he has new duties to occupy him.
To use a weird analogy, you and your friends are at a party, and Sue notices that Bob isn't around, like he always used to be. At which point you remind Sue that Bob has a kid these days so he doesn't party as much as he used to.
Now, of course, Sue knew Bob had a kid. She's just used to Bob being part of the scene and hadn't quite processed that he's got other things going on in his life these days.
When Kirk stresses "
Captain Sulu" what he's really saying is
"Captain Sulu, remember?"
As in: "Don't forget, Bob has a kid now."