*Sigh* There ARE other crux points in history for creating alternate timelines besides the Revolutionary War, the Civil War, World War II and Kennedy's assassination, sci-fi writers. :/
No, this is not a failing of oversight that's unique to Moore, but then, that's kind of the problem in and of itself, isn't it?
When even a BRITISH writer winds up going where roughly sleventy baskrillion Twilight Zone episodes have already gone before, we're BEYOND Old!Meme territory.
Even as an American, one of the things I loved about one of the original series episodes of Doctor Who was that it showed the Master trying to fuck with Earth's history by preventing the signing of the Magna Carta. THAT'S an interesting divergence point, and one that HASN'T been done to death.
I mean, if your story about the South winning the Civil War exists simply to show us HEY LOOK BLACK PEOPLE ARE SUBSERVIENT AND OPPRESSED AND THERE ARE CONFEDERATE FLAGS INSTEAD OF AMERICAN FLAGS AND OMG ISN'T THAT TERRIBLE, you're not exactly charting any new courses of imagineering, writer-guy.