Moridin aside he's the only one who has any slight mystery around his where-about's as all the others have been publicly balefired. I'm sure Demandred has been told about Asmodean being out of reach of even the DO but in a moment of panic (or as close as Demandred gets) the thought process of 'not from this age' -> one of the forsaken -> not Moridin -> 'could it be Asmodean isn't dead after all?' isn't that much of a stretch.

Saying that, while I think that's probably the logic behind the statement, I don't think it's a great piece of writing.