My guess is that Cleon I (himself via the chamber, or via some sort of dual-personality presence with Demerzel) has been racking up memories this whole time because he isn’t truly gone.

Perhaps Empire had some analogue to the Prime Radiant, where his consciousness is tied to Demerzel in exchange for her continued existence.

From what I gather, each clone only gets their 75years of memories before they are put down for the next in line. With memory editing, they’ve slowly been getting smaller and smaller memory banks.