To seriously voice such a wish, IMHO, would have implied that Sulu somehow knew that the Excelsior did not have a captain (anymore) by the end of the film.
Why? It's not a real-world requirement that a position be empty in order for a new guy to move into it. Except perhaps in certain very rare cases where the person in the position cannot move further up, cannot move laterally, and isn't going to retire quite yet. But in Starfleet, that would be the top flag ranks (and even there we see "lateral" reshuffling for Cartwright), not mere starship commands.
In interpreted this as, not necessarily that Styles was now gone, but that it was known that he was only occupying the Captain's Chair on a very limited and short term basis. Maybe instead he was stationed at Spacedock and was only in charge of the initial trials (which may have been completed in the 3+ months that passed since TSFS). Making Styles a sort of glorified valet / test driver. Yes, he did posess the RANK of captain, but as has before in TOS, not everyone with the rank is suited for command.
Never thought about this until now, but we never see Styles refer to one of his crew by name. Its always a general positional reference, "Bridge" or "Helm". Since the ship was soon to undertake its trials, the fact that Styles doesn't seem to have developed more of a relationship with his crew, or selected a crew that he would do so, suggest he is not expecting to stay. By this, I am not expecting a warm lets play cards with my crew like in all good things, but something at least something less than the borderline contempt we saw "How can you have a yellow alert in spacedock?"
In fact, he had more familiarity with (or contempt for) Admiral Kirk, who had not been in active command of a starship, but working at Starfleet Command for some time.
Additionally, when Excelsior's drive breaks down, Styles expression seems to convey more annoyance at how the situation affects him, than how it has affected the ship. Granted, starship captains can have egos too. But some concern about the status of his ship I would expect to weigh in. Maybe a quick frantic call to Engineering. Then again, Scotty's loyalty to Kirk, he may have quickly surmised that Scotty sabotaged the ship and made of with the Enterprise.
A problem here is trying to evaluate motivations of characters based on behaviors that the filmmakers admitted were deliberately skewed and exaggerated in order to make the Enterprise crew look more professional in comparison. So in light of this, I try to account and compensate for this "narcissistic lens" that casts over the rest of starfleet. My conclusion, Styles is not an idiot, nor is he a complete pompous ass. He's likely a competent career officer serving a function as temporarily in command of Excelsior during the transwarp trials. He knows it, his crew know it, and likely so does most of people familiar with the Excelsior. Ships in the US Navy often have multiple commanders, there is no reason the same couldn't be true for Starfleet. For all we know, Styles may in fact have his own ship.
If we go by the assumption that Sulu was originally meant to be her captain (from cut dialogue in TWOK which admittedly isn't cannon), it is likely that the political mess that was left in the wake of the Khan incident put that on hold. Starfleet then would have to scramble to find someone to conduct Excelsiors trials. Making this another sort of Jelico situation: another Captain with his own ship and crew temporarily assuming command. As debated in the Chain of Command
thread, the behavior we see likely is very different from how he handles his own crew.
Further support -- With all of Scotty's resentment and complaining about Excelsior, we do not see any of that directed toward Styles. Certainly Scotty seemed cordially indifferent to Style's remark about breaking Enterprise's Speed Records. But his beef seemed more about the SHIP and the people who designed her, and Styles is just one of the many players in the political situation.
The whole first act of TSFS has so many disconnects in tone from TWOK (not necessarily continuity errors), that there seems to be a lot of political subtext going on. Granted, much of this disconnect comes the reality of production; from introducing elements that would seem like they would have been seen in TWOK but didn't because they hadn't yet been conceived by the filmmakers.