Haha. You and several others have made good points about how vague and inconsistent it has been in the past already. I do believe that some folks have been overthinking the situation, which is par for the trekbbs course. But I know it can be fun to squabble over the details.Hell, DS9 once said the Eugenics Wars took place around 2170 and that set off a lot of fans. That episode was more than 25 years ago and somehow we all survived.
Not every incongruity or retcon has to be evidence of a separate timeline. There has been plenty of time travel in Trek, and sometimes the fiction intends for the viewer to interpret events as a new timeline, but more commonly the existing timeline just gets tweaked and the viewer is expected to believe that everything continues on pretty normally regardless. In this case, the Prime timeline remains Prime, and there's no need to imagine some dramatic divergence. Change two words of Space Seed in your head and we're golden.
Star Trek isn't real, the butterfly effect is only in play if a writer wills it to be so, so if writers want the details of the eugenics wars to be a little flexible while maintaining existing continuity, they can. And we can all carry on from that per usual business.