I don't mean that a flashback story wouldn't happen, I meant this particular story wouldn't have happened. Far as I can tell, Waid isn't paying much attention to what continuity is supposed to be and is just having fun with the retro Silver Age-y vibe, and the book is much better for it. For example, as far as I can tell, looking at what continuity is supposed to be today, Kara wouldn't have been around when Dick was Robin. But she's there in Waid's story, and it's great.
I agree that locking every single story into a single timeline is impossible, and not worth the effort anyway. How often do stories like Millennium Giants get referenced or matter to the stuff happening today? Not worth trying to fit it all into a timeline. I figure 90% of stories end up as fluff and dross; they might be fun to read, but there's little point in trying to cement them into position within a history because they don't carry enough weight to matter.
But DC could take the major stories and developments and work it all into a single framework. And really I wouldn't want anything more involved and complex than that basic foundation. Give us enough to know the general shape of the history without getting bogged down in the details.