Not the specific phrase “Everything Happened”, no; I was conflating that with the very similar phrase “It's All True”. See the following article, published way back in the late 90s immediately after Hypertime was first introduced:
Waid on the Keys to the Kingdom.
I'll grant that unlocking memories of past iterations is new; but the idea of giving the writers a way to incorporate anything from DC's past into their stories
isn't new; and the whole point behind the concept of Hypertime was to enable this.
In particular, the most controversial aspect of Hypertime is its ability to temporarily or permanently
merge alternate timelines, letting the writer incorporate elements of a previous story that had long been declared “out of canon” — very much the same sort of thing that the “memories of past iterations” notion you're talking about is trying to enable. It's arguably even more powerful; possibly
too powerful: in theory, it can allow you to bring back
anything ever published. Compared to that, the ability to split the timeline at a decision point is tame.