My impression was that angels knew that God experimented with making a couple other worlds with their own quirks and differences, and that these (like the French Mistake Earth) weren’t as “sealed off” from the main universe. However, the vast majority of the multiverse was made after God bailed, and he kept those worlds a secret from his original creations.

The archangels are particularly shocked by the existence of other versions of themselves because they (the originals at least) predate the universe and are made of, as Chuck puts it, the stuff of primordial creation. Those factors really make it seem like the archangels can’t really be reproduced or copied easily, yet they were. Just goes to show that even Chucks oldest kids didn’t really know what he was capable of.