I think he simply means an overarching era from start to end.
None of new AU reach same length as UC so in a sense it looks like AU failed establishing something.
Looking again, without context, doesn't Witch of Mercury sounds "Chapter 2"-ish title when put side-by-side with Iron-Blooded Orphans? Not surprising if due to that some think 'maybe this indicated continuation to IBO era like Destiny to SEED (or maybe Z to ZZ)? Why no past AU even tried like this smh.'
