I don't think there's a rule, I think there was a confusion during the separation of Viacom that led to this rumor, but it's never been said because I think they never intended to separate the franchise. Reading that reddit thread seems to suggest that it was confusion by the producers talking to management and the producers erred on the side of caution although it was what they wanted anyway so didn't pursue it further.
I know they originally separated the license between the individual parts of the franchise, that's why the games were split between TOS and TNG+DS9+VOY, for example. Activision had first right of refusal on series that came after TNG and since they were all of the same time period it was easy enough to manage. But they unified that license after Nemesis and they intended the Kelvin timeline to become the main one, while the original timeline remained under their publishing licenses not TV licenses. IDW published comics in both timelines for example without any kind of scheduling hitch, they simply added Kelvin books. (The reimaginings of classic TOS episodes actually were some of the best Kelvin stuff I thought.) Pocket Books was going to similarly add Kelvin but it got "cancelled" before they got around to it so they canned or reworked in regular TOS the two books that were scheduled.
When Brian Fuller came back he had zero problem with his anthology series that would cover the entire breadth of the franchise.
Star Trek Online has been a unique case for some time, that I don't know that looking at for answers will get you any. They originally were going to stay separate from Kelvin, but then gradually added some stuff and reworked their own timeline to fit it in. Pocket Books timeline was entirely separate and also as "canon" as Online until they decided to junk Kelvin.
This isn't a case like Sony/Marvel and Fox/Marvel where it was literally two different companies with one holding the original licenses and one holding the film rights, this was one company split into multiple parts. National Amusements, the vehicle for Summer Redstone's estate, still owns a majority of both the new Viacom and the new CBS. I'm pretty sure they, acting as the chair of the board and majority, could at any time simply sign off on anything if they needed an official legal paper trail.
