At the time, the timeframe of ST hadn't been locked down yet. Looking at it retroactively, it clearly doesn't fit what's now been established about the chronology, but it's easy enough to assume that Decker's memory of historical dates was imperfect and he misspoke.
Right. The first time Star Trek
is actually stated to take place in the twenty third century is in TWOK.
Though IIRC, in The Making of Star Trek
, Roddenberry often used the 23rd Century as a reference point when talking about the future. So it's interesting that it was chosen to be the setting of Star Trek in TWOK.