A few potential theories:
A) GR simply got the name DEFIANCE wrong
B) Some research assistant got the name wrong and it made its way into the Writers Guide that way, and no one ever noticed.
C) GR simply liked DEFIANT better than DEFIANCE
D) Some brilliant forward-thinking young guy from the Rand Corp Think-Tank already knew the assaultive "T" sound at the end of DEFIANT would sound MUCH better through a little 4" TV speaker than the historically accurate DEFIANCE, with its soft and airy "S" sound at the end.
Here is the theory I like the best:
Someone, not Coon or GR, in thinking about continuity foresaw the need to create a detailed list of these "other starships' in the fleet after someone submitted a story which used the names of other starships (out of the writers imagination)- for the reference of all future writers, and so forth,... Coon said: "Good, you generate it", the "kid" wrote it from his memory of 7th Grade British History - using the name DEFIANT - showed it to Coon (who had already forgotten about the minor thing), told the "kid" to get it okay'd by GR, GR glanced through it with the 'good old executive once over', saw no conflicts or problems, and said: "fine, fine, so that will be the name of the other ships, thanks." The "kid" took it to mimeo, and had it inserted into the Writers Guide as "Cannon",... Sounds "logical" to me LOL!
First off: This was the third season, at which point Gene Coon had left the show and Gene Roddenberry had stopped participating in the writing process. Neither of them would've had anything to do with the selection of the name
Defiant. If it was changed from
Scimitar to
Defiant in rewrites, it might've been Fred Freiberger, Bob Justman, or story editor Arthur H. Singer who was responsible for the change (assuming it wasn't the credited screenwriters' idea).
Second:
The Making of Star Trek reproduces (on pp. 163-4) a memo that D. C. Fontana sent to Roddenberry and Bob Justman after "Tomorrow is Yesterday" established that there were 12 ships like the
Enterprise in the fleet. She offered 22 candidate names. In reply, Justman narrowed the list to 11 of Fontana's candidates and added one more (
Saratoga, as it happens). Not all of them were used, and other names were introduced in various episodes. Neither list included
Defiant. Although Fontana's memo did suggest putting these names in the writers' bible, we don't know if that was ever done. The copy of the bible that's generally available is from April 1967, before Fontana's memo. However, the
Phase II bible from 1977, which is largely just an updated version of the TOS bible, does not contain a list of ship names, so it seems likely that the TOS bible never did.
And really, a list of names like that is not the sort of detail that you'd include in a writers' bible. A bible is a primer on the fundamentals of the show -- the characters, the premise, the world/setting they occupy, the basic terminology. An aspiring screenwriter for TOS would've needed to know what a phaser or an impulse engine or a briefing room or a Vulcan was, but they wouldn't have needed to know the names of all the ships in the fleet, because that was a detail that could easily be invented by the scripter or tweaked in rewrites by the staff.