Actually, she spoke of finding out a secret. It does not sound like she was simply told about it. Maybe she overheard Moridin bragging abou this to Cyndane in TAR as she was answering his summons or something. Maybe she somehow managed to get it out of Cyndane, but that's more doubtful.

In any case, it doesn't matter much. This is the last book and obviously a plot device to tell us the final missing piece of what happened to Cyndane (what could remain is a reflection from Moridin about Cyndane, or from Cyndane about Moridin that would tell us more of their grudge. As I said, it's pretty obvious Moridin hates Cyndane, because he's the one she challenged in TDR, and he lost and died, and she went on triumphant to go on with the fiasco she called a plan).

We could ask Sanderson, but I'm pretty sure we have to take what Moghedien thought of those events at face value unless there's more to come, but I doubt it.