I think it's also arguable that we are supposed to believe that the uniforms, ship, more nautical trappings, etc. haven't changed from TOS either, instead of being eleven years out as from (to quote Decker) "a totally new Enterprise", she's now an old training ship, Khan wears a broken belt buckle from a supposedly "modern" officer's uniform around his neck.
Plus apparently there was a cut scene where Khan would comment that the Reliant's bridge was just like the Enterprise's bridge which implies that as far as the movie was concerned the Enterprise always had the TMP bridge.