To be fair, the Okuda's did preface their book with a piece basically explaining their policy on conjecture and how it isn't meant to be taken as definitive. And an astonishing amount of the book's entries have the tag 'Conjecture' next to them, making them suspect.
Yes, but everybody always ignores that part and takes it as gospel anyway. And the tie-in authors are required to accept the Chronology's conjectures as fact unless they're overwritten by subsequent canon.
Generations explicitly dated Kirk's time with Antonia -- during at least part of which he was retired -- from 2282-84 (11 to 9 years prior to the Nexus incident in 2293). That pretty much rules out TWOK/TSFS/TVH/TFF taking place during that span. And TWOK can't really come earlier than 2282 (because that's 15 years after "Space Seed"), so it has to be after 2284. The most logical place to put that relationship and retirement is before TWOK -- Kirk tries a life with Antonia, it fails, he goes back to Starfleet, and in the wake of that he feels old and depressed and trapped by his career. So to me, it pretty much verifies the Okuda dating of TWOK as 2285, however much I wish that weren't the case.
I had an idea so I made a trip to a book shop to have a look. The recently published Autobiography of James T Kirk has this to say on the matters post TMP (this is a pretty rough précis): Kirk says, and I quote, 'after our second 5YM ended', Kirk then decides to resign from Starfleet rather than be promoted again, he travels and moves onto a farm before eventually going back prior to WOK! I know it's not strictly canon but where does the official autobiography fit?
None of the tie-ins are any more "official" or binding than any of the others. They're all equally conjectural. (They're all official, but that just means they're published with the contractual consent of the property owner and are being sold legally.)