All it means is that none of the other previous captains are not admirals on Earth at that time
Am I misreading this? Kirk was qualified despite
holding flag rank, so we do have to count people promoted to Admirals.
And his "year class" of starship skippers is not the only one we have to consider: the movie is evidence that the previous
commanders of starships did not gain experience on five-year missions against V'Ger like things, either. It's not just twelve people (minus casualties) we are discussing. If assignments lasting five years are exceptionally long, then it's all the more reason to think there are dozens upon dozens of former starship skippers available at Earth, and yet none can outshine Rear Admiral Kirk. (If five-year missions are
common, then there might be fewer ex-skippers in existence, but the odds of finding an experienced skipper on Earth as opposed to an inexperienced one would correspondingly increase.)
The next movie already blows out of water the idea that you need one of the dozen Constitutions
in order to clock up a five-year mission; the technical readiness for that is available in at least one other type of starship, and nothing indicates it wouldn't be in dozens or perhaps hundreds of types. Add to that ship types preceding the Constitution
but still supposedly serving (why would there not
be overlap?), and you have preempted the concept that starship availability limitations would prevent one from holding five-year-mission experience.
All this thus dovetails to five years being a really big thing for nuKirk in ST:ID...