It wasn't addressed in TNG. It was touched on in DS9 and then elaborated on in ENT. But it's an explanation that is much more interesting than what they did in ENT. I seem to recall it' became the fanon explanation after TMP was released.
But aren't the books suppose to be about the production of TOS? Using various production notes to give us an accurate history?
I can make up anything I want and pass it off as fact as well.