Frankly, I didn't feel too comfortable actually doing it, but since most of the people that could have provided an answer are unfortunately no longer with us and since the lingo and terminology in TOS, TMP and TNG is apparently the same (e.g. "intermix formula"), it seemed okay to go to TNG and look for conclusions that could help to find answers for TOS.
Since it doesn't contradict or "rewrite" TOS I think it's fair to say we are not looking at a ret-con maneuver.
Alternately, we could arrive at the same conclusions just by looking at the lingo used in TOS. The Making of Star Trek
refers to the "fusion" of matter and antimatter and then there is this strange "integrator" mentioned in "That Which Survives".
As a matter of fact, when you bring matter and antimatter together these don't fuse or integrate (that could apply to fusion power) but they annihilate each other which is anything but fusion or integration, but quite a euphemism (it's like saying the H Bomb is a "natural" weapon because it works like the processes in our sun which give us warmth and life).
If you were to add, mix or integrate fusion energy into the annihilation energy the words "fusion" or "integrator" would at least make some sense.
And then there's the apparent necessity to preheat matter and/or antimatter according to Scotty in "The Naked Time". Take the extremely hot ionized plasma resulting from nuclear fusion and you can be certain that the automatic magnetic confinement mechanisms in the reactor chamber will instantly activate and thus be prepared for the coming bigger blast when matter and antimatter particles start their annihilation business.
P.S. Just had a déja-vu when rereading this comment. I think someone is about to be coming to take these ideas apart...