On the other hand, REtroactive CONtinuity would imply taking something from a subsequent series and applying it to TOS, wouldn't it?
Yes, that's exactly what it means and what TPTB and a lot of fans have been doing ever since there were spinoffs of TOS. It was tolerable until the films where it started to get ever more twisted.
Some retcons can work when it can explain something vague in TOS yet without violating or being inconsistent with the overall subtext. But when it is in direct contradiction of what was explicitly said, shown or inferred in subtext then it's blatant and (in my opinion) wrong headed.
Example. In "Space Seed" Spock says WW3 and the Eugenics Wars of the early 1990s were one and the same thing. Yet in TNG it's rewritten as separate conflicts. From then on practically everybody goes with the explanation that Spock got it wrong and misremembered his history. Oh, and Kirk and McCoy happened to misremember theirs as well?---because they sure as hell didn't correct Spock. I don't think so. And this is but one of many outright and more subtle contradictions that piled up over the course of several films and spinoff series.
It's understandable on some level because a lot of fans want all the films and series to be of one consistent continuity. However, another way to look at it (to each his/her own) is that each subsequent spinoff was an alternate continuity that shared many similar elements yet differed (sometimes greatly) in others. And this can include TAS and the films as well each according to your own perception.